Okay I think I understand a little better now. If the main question is “is there a tool for automatically updating code (i.e. syntax) to newer Julia versions” the answer is no as far as I know.
As I said, Julia follows SemVer so code written for 1.0 should run on Julia 1.8. I think the main issue you might be facing is with breaking package updates - you can of course continue using the old package versions but that will likely mean missing out some of the benefits of e.g. better threading support in later Julia versions.
Code for 0.3.12 is unfortunately not easy to update - there were 4 breaking versions (0.4, 0.5, 0.6 and 0.7) since then. 0.7 is actually the same as 1.0, but with deprecation warnings instead of errros for changes between 0.6 and 1.0, so it’s a sort of “bridge” to allow users to update more easily. You can either try going straight from 0.3 to 0.7 (and thus 1.0), or go version-by-version if too many things break because of such a large jump.
In any case I would assume that people round here will be happy to help if you run into trouble.
Finally, on
So, what will be the best Julia version to implement?
I think your choices are to target either current stable (1.8.1) or LTS (1.6.7). Here’s a read on Julia’s release process including a discussion of “risk tolerance personas” and recommended Julia versions: