From our experience. 0.11 -> 0.12: We're not attempting. We're in the process of changing out config management anyway and we're unhappy with a bunch of decisions in that terraform stack, so it's a good point to get rid of it.
0.12 -> 0.13: Well, we had to add what feels like a million required-provider-blocks and then some more. And sometimes it was tricky to pinpoint the module pulling in a default provider and crashing - `terraform providers` and `terraform graph` help there. The graph is easy to grep through to find the resources and modules pulling in wrong providers. And in the beginning the error message we got when we had to run 'terraform state upgrade-providers' was .. obscure. In newer versions, that message is much better.
0.13 -> 0.14 just happened and now the lock files are slowly piling in on demand.
0.12 -> 0.13: Well, we had to add what feels like a million required-provider-blocks and then some more. And sometimes it was tricky to pinpoint the module pulling in a default provider and crashing - `terraform providers` and `terraform graph` help there. The graph is easy to grep through to find the resources and modules pulling in wrong providers. And in the beginning the error message we got when we had to run 'terraform state upgrade-providers' was .. obscure. In newer versions, that message is much better.
0.13 -> 0.14 just happened and now the lock files are slowly piling in on demand.