I recommend a few things.
Migrate in steps with testing after each.
Level 1: If you are currently using Rule Team Server, you will be migrating the repository to Decision Center. Keep the classic rule projects, enterprise console, and classic rule engine.
Level 2: once at Level 1, migrate classic rule projects to decision services (keeping the classic rule engine).
Level 3: once at Level 2, switch to the decision engine. If you have a bunch or IRL code, this is converted to ARL with the decision engine. It isn't always a 1-to-1 mapping.