Installing casters will not void your manufacturer's warranty. However, please keep in mind the your Line 6 manufacturer's warranty is 1 year from the date of purchase.
If oyu purchased an extended warranty through your dealer then any issue that comes up after the 1 year manufacturer warranty will need to be handled through them.
Has this answered your question?
It's been a couple of days since we've heard from you. We're going to assume your question has been answered and close this thread. If you still have questions please feel free to open a new question with us and we will answer your questions accordingly. Thanks!