there doesn't seem to be a change or fix to have any benefits to paying off a lease/loan early.
if you take a lease/loan and attempt to pay the loan off early, there is no adjustment for the amortization of the loan.
my suggestion is to have a simple 'credit rating' for the initial loan based on debt (existing leases) to equity + future income (maybe a small adjustment for diplomacy). when a lease is taken out on that turn, there is a interest rate associated w/ the credit rating. if a loan is paid off early, it should only be the principal amount and maybe a small early pre-payment fee; while the remaining future interest payment should be removed from the early payment of the loan.
otherwise pre-paying off a lease early has no benefit and 1x the loan is taken out, there is no reason to ever pay it off early.