Changing to AHCI shouldn't cancel your warrenty, and I think your warrenty won't be affected whatever you change in your "aspire one" cause there are no advanced changes you can do there anyway.
If it has AHCI then it is OK, we want AHCI, the problem is IDE not AHCI.
What we want is to make the hard drive run in AHCI mode instead of IDE. But I'm not even sure that the problem is comming from there, all I'm saying is that it would be the first thing I would check if I have to repair a slow aspire one.
I don't have any aspire one arround to see the exact steps you need to follow to check whether it's running with AHCI. All I know is that you can access bios config by clicking F2 right after you turn your computer on.
Sorry reyne, I don't know how to help you more.
Last thing, take a look at this picture, I found it on google, it may help you :
http
/1.bp.blogspot.com/_qNFgsHp2dCk/S45hBDdM3iI/AAAAAAAAC2s/SyTW3ecOPcI/s400/Aspire-4740G-BIOS-SATA-IDE-mode-large.jpg