I would suggest contacting GBCI directly about any potential loss of documentation. I believe some will be lost, but GBCI will be able to more specifically answer that question.
Although the base credit requirments do not change from form version to form version, there are different documentation requirements for each version. Some make it simpler and easier while others add documentation requirments. The only way I know of to determine the benefit (or not) of upgrading is to look through the form versions.
I will caution you that many beta forms have glitches and that even when you don't upgrade the forms you may be asked for documentation that is required on later versions.
You are correct, it is not required to upgrade your forms.