You will want to use Documaker Studio even if you are currently using one of the prior design tools for your forms. There really is no 'migration' per se as your form should work from one version to the next of Documaker without editing. If you do run into issues you should contact customer support via the Oracle Support site.
You haven't provided enough information to determine what you currently have and what you now expect to do. Technically, there may be nothing special to do to upgrade besides testing your output versus what you had before. If you encounter a problem, then you will have something you can report here or to Support.
The best thing to do is treat your upgrade like you would for any other project. Have a project plan, and in it plan to test Studio GUI functionality, set aside time to thoroughly test all DAL scripts, test all triggers, and test any special rules you have on sections, or within afgjob. If you have many different products, you will want to test each one independently since you may have different functionality in each. One product may convert without problems while another may have minor anomalies you would have to re-configure.
You will encounter anomalies which you can then report to Oracle support. What you thought was a feature in 11.3 may actually be a discrepancy that was fixed in 12.x. That's what we encountered when we upgraded, we were told we took advantage of a problem that ultimately got fixed. Over all our upgrade is a success.