Migration Legacy Symfony ​ in Production​

When inheriting an unmaintained legacy Symfony application, the real question is not “how to migrate”, but rather “where to start without breaking everything”.

In this talk, I share an honest retrospective of a Symfony migration carried out in production, on an aging application with limited documentation, few tests, and dependencies on components that are now obsolete.

Through this journey, I offer a reading of the battlefield: risk areas, false good ideas, and decisions that seem obvious on paper… but turn out to be dangerous once confronted with real-world constraints.

In particular, we will cover:
- How to step back from an unmaintained legacy codebase and identify the real problems
- The false good ideas we encountered during Symfony legacy migrations
- Common mistakes observed in the field, and how to avoid or mitigate them
- Small levers that make a big difference (order of actions, scope, tools, trade-offs)
- Key lessons learned from this migration: what we would do again, and what we would clearly avoid

This talk is aimed at developers working with inherited Symfony applications, often business-critical, who must move forward with limited visibility and very little room for error.

The goal is not to promote a perfect or universal method, but to share a real, lived experience, and provide attendees with concrete reference points, clarity, and practical tips they can apply to their own migrations.

Transcript

2. Symfony 7.4 Symfony 3.4
3. Dead code Symfony 3.4 Symfony 7.4
4. Dead code Conflicts Symfony 3.4 Symfony 7.4
5. Dead code Conflicts Production pressure Symfony 3.4 Symfony 7.4
6. Dead code Conflicts Production pressure Decision Symfony 3.4 Symfony 7.4
7. It is not side work
8. It is engineering
9. This talk is not about SCOPE
10. Ultimate guide This talk is not about SCOPE
11. Ultimate guide Toolbox This talk is not about SCOPE
12. This talk is about SCOPE
13. Real feedback This talk is about SCOPE
14. Real feedback Our journey This talk is about SCOPE
15. Real feedback Some pain Our journey This talk is about SCOPE
16. Real feedback Some pain And victories Our journey This talk is about SCOPE
17. The situation we inherited SCOPE
18. The situation we inherited Multiple applications SCOPE
19. Multiple applications Different starting points The situation we inherited SCOPE
20. Multiple applications Different starting points The situation we inherited One carries it all SCOPE
21. Stakeholders 01/10
22. Same language Stakeholders JOURNEY
23. Same language Risks Stakeholders JOURNEY
24. Same language Performance Risks Stakeholders JOURNEY Thursday, June 11, 2026 at 8:00 AM America/Toronto 90 minutes
25. The rules of the game 02/10
26. No code freeze The rules of the game JOURNEY
27. No code freeze Continuous feature delivery The rules of the game JOURNEY
28. The Plan 03/10
29. Define our process loop The plan JOURNEY
30. Define our process loop Deprecation free The plan JOURNEY
31. Define our process loop Deprecation free Increase version The plan JOURNEY
32. Define our process loop Deprecation free Increase version Test on preprod The plan JOURNEY
33. Define our process loop Deprecation free Increase version Test on preprod Deploy The plan JOURNEY
34. Define our process loop Deprecation free Increase version Test on preprod Deploy The plan JOURNEY
35. Everything looked great on paper
36. ... until it didn’t
37. Dead code 04/10
38. Dead code PAIN of the code we carefully migrated was never used in production. 35%
39. Double the effort 35% code migrated for nothing Dead code PAIN
40. Clean code before migration Double the effort 35% code migrated for nothing Dead code PAIN LESSON
41. Testing effort 05/10
42. No documentation Testing effort PAIN
43. No tests No documentation Testing effort PAIN
44. No tests No documentation Assumptions instead of verification Testing effort PAIN
45. No tests No documentation Test on critical flows Testing effort PAIN Assumptions instead of verification LESSON
46. Bundle dependency 06/10
47. Bundles became incompatible Bundle dependency PAIN
48. Heavy dependency Bundles became incompatible Bundle dependency PAIN
49. Heavy dependency Bundles became incompatible Abstraction layer Bundle dependency PAIN LESSON
50. Branch war 07/10
51. Branch war PAIN 5 1 v s Five devs shipping features One dev migrating
52. Each day = conflicts 5 devs vs 1 dev Branch war PAIN
53. Each day = conflicts 5 devs vs 1 dev Deliver small pieces as early as possible Branch war PAIN LESSON
54. Take a decision 08/10
55. Cannot change everything at once Take a decision PAIN
56. Making choices takes courage Can't change everything at once Take a decision PAIN
57. Plan the rest for later Take a decision PAIN LESSON Can't change everything at once Making choices takes courage
58. Some victories 09/10
59. Some victories VICTORY 0 code freeze
60. Some victories VICTORY 0 code freeze 8 months Migration and delivery
61. Dockerization And tools VICTORY
62. Dockerization Rector And tools VICTORY
63. Dockerization Rector PHPStan And tools VICTORY
64. Dockerization Rector PHPStan Symfony GitHub And tools VICTORY
65. And today 10/10
66. Symfony 7.4 And today TODAY
67. Symfony 7.4 PHP 8.4 And today TODAY
68. Symfony 7.4 PHP 8.4 Not going back to Legacy And today TODAY
69. What I wish I had known Takeaways
70. What I wish I had known TAKEAWAYS Clean code before migration Spend time before not after
71. What I wish I had known Clean code before migration Test on critical flows Spend time before not after Know what cannot break TAKEAWAYS
72. What I wish I had known Clean code before migration Test on critical flows Deliver small pieces early Spend time before not after Merge often, conflict less Know what cannot break TAKEAWAYS
73. What I wish I had known Clean code before migration Test on critical flows Deliver small pieces early Don’t chase perfection Spend time before not after Merge often, conflict less Stay focused on the goal Know what cannot break TAKEAWAYS
74. Clean code before migration Test on critical flows Deliver small pieces early Don’t chase perfection Spend time before not after Merge often, conflict less Stay focused on the goal Know what cannot break Use their language Frame risks, not code What I wish I had known TAKEAWAYS
75. It is not side work
76. It is engineering
77. Arnaud Oltra www.linkedin.com/in/arnaud-oltra/ Migration Legacy Symfony in Production Tech Lead