If loan repayments are fixed then early repayment might not result in recalculation of the loan - i.e. you'll pay it off sooner than the expected term but the repayment will remain calculated on the basis of it being 20k being paid back over the term.
So for relative demonstrative purposes (i.e. figures aren't specifically accurate), if borrowing 20,000 over x years at 6.8 works out at say 600 a month, but borrowing 10,000 over x years at 8.7 works out at say 350 a month, if you take out the 20k and immediately pay back 10 you'd still be paying 600 a month rather than 350 a month. The interest rate will be better and you'll pay it off quicker but the repayments will be significantly higher in liquid cash payment terms.