By contrast, over the last 8 years I've migrated a number of small micro services up from every version of Scala since 2.10 with no problems, maybe some minor d

By contrast, over the last 8 years I've migrated a number of small micro services up from every version of Scala since 2.10 with no problems, maybe some minor dependency hell with some of them. I've even upgraded a simple project to Scala 3 with no code changes. It depends on what libraries you use. The big hurdles tend to involve macros and Spark.