I've been using QB forever, my files are quite large... how can I transfer to QB on-line for future usage
Well you cannot effectively transfer such a large file with years and years of data. If you have a paid for edition of QB Desktop you certainly can always open your up to conversion set of data for review or taxes or whatever.
The solution for you may have several steps and may depend on what year your QBDT is.
Things to do, not necessarily in exact order.
1. back up current desktop company
2. create a copy of current desktop company
3. condense copy of desktop, which will turn years and years of historical data into a journal entry