Early/late return penalties?
 

I have a 3-day rental reserved in Ljubljana, Slovenia. Depending on circumstances, I may wish to have the car for a few more days. What is my best strategy:

1) Modify my existing 3-day rental to a 5- or 6-day rental and return it early if I only need the 3 days, or

2) Cancel the existing 3-day rental and make a new 5- or 6-day reservation, and return it early if I only need the 3 days, or

3) Keep the existing 3-day rental, make an additional 5- or 6-day reservation, and cancel the one I don't need once our plans are finalized, likely only a couple of days before the rental commences?

or something else?

I assume that if I take option 1 or 2, there would not be any early-return penalty other than the fact that the longer rental would be a weekly rate and converting it to a 3-day rental would make it a higher per-day rate since it would no longer be a weekly rental. But assuming could land me in trouble, which is why I'm asking here. If I convert either to a longer or shorter rental at the time I pick up the car, would the rate be higher than a comparable advance reservation?