Short answer I cant submit an entry without ticking boxes which then requires full payment?
Use the EFT/cheque entry form not the Paypal version.
Yes, tick the box as full entry and it will show full charge and that is correct as you are entering. Now at the bottom of the entry form there is a comments section in there just type that you have a carry over entry fee amount of $X from last year and therefore you will only pay $10 if entry only or $15 for entry and one event licence, depending what the carry over amount is as well.
The system will ask for a full entry EFT to be done but you should do an EFT for the small amount you now owe and please put your name in the description part of the EFT process so we can track it.
Hope that is reasonably clear
. It is difficult to have online forms for every possible permutation that could occur.