Upgrading Terraform code from 0.13.5 to the latest version

I've inherited a Terraform code base that is outdated. It's currently running on 0.13.5 and inherits Terraform modules that I guess were written specifically for 0.13.5.

There is not that much info I can find regarding on what the complications could be when upgrading the code base from 0.13.5 to the latest (1.3.9).

Would I need to change anything in the modules that get pulled in?

Would it better to upgrade from 0.13.5 to 0.14+ then on to 0.15+ etc?

Would I need to change the version of the AWS Provider? That is currently 3.9.0 across the board.

Please feel free to share your suggestions on what the best approach would be.

As per the documentation:

> If you are currently using Terraform v0.13 or earlier then we strongly recommend upgrading one major version at a time until you reach Terraform v0.14, following the upgrade guides of each of those versions, because those earlier versions include mechanisms to automatically detect necessary changes to your configuration, and in some cases also automatically edit your configuration to include those changes. Once you reach Terraform v0.14 you can then skip directly from there to Terraform v1.0.

Note the following as well:

> You can also upgrade directly from Terraform v0.14 if you wish, although please still consider the notes from the Terraform v0.15 upgrade guide. If you are affected by the notes in that upgrade guide, you will still need to take the steps described there but you can do so as part of upgrading to v1.0, without any need for an intermediate step of running Terraform v0.15.

For upgrading from 0.13 to 0.14, you can follow this guide.

The provider version should be good as long as you do not have too many resources which are using S3 buckets, so prior to upgrading make sure to read the upgrade guide.