The TP currently has an expiration date of April 15. With the January 21 release of the TP, there could well be a new expiration date. We just don't know at this point.
In the past, no matter which product was being tested, the Beta, TP, whatever it's called, must be uninstalled before installing the final product, sometimes called RTM (Release to Market). One thing to note is that Microsoft is getting better and better at this; it may well be that we'll be able to just upgrade, but don't count on anything to be written in stone.
However, if it were me, I would not replace my Windows 7 or 8.1 OS with Windows 10 at this point. As Jeff said, you can always install on a VM (virtual machine).
I second that, Mystere! Upgrading (especially from a TP/Beta or even Windows 8.1) may be convenient, but you stand a huge chance of including some of the things (the sleep problem in Win 8.1 comes to mind) you hated.
Back in the dim and distant past when W7 was in beta / trial the trial was extended by an update for a few months.
If W10 is to be released in Q2 / Q3 / Q4 of 2015 its probably likely that there will be an extension or at least an updateable new version with a later expiry than April.
Of course all bets could be off on this one - but as so many people seem to be running SUCCESSFULLY W10 on a daily basis (even though advice was given against people doing this) Ms will possibly want to avoid mega aggro if all these people have to revert to their previous OS or re-install all their programs again.
I've been very pleasantly surprised at the stability of W10 - considering it's pre-beta even and (again all things being equal) will ditch W8.1 in favour of it if the cost isn't horrendous.
My main faults with W10 have been essentially when running it as a VM -- Unity mode is "Flakey" but can be made to work and USB3 support causes the VM not to be able to attach USB devices - whether that's due to the HOST USB 3 support or W10 or VMWARE I haven't a clue but running with USB2 mode works just fine.
I still see a lot of diehards on the W7 Forum who seem unwilling even to give W10 a try as a VM - well it's their choice of course.
I can't wait to see the CONSUMER preview to see what Touch enabled features have been improved but I've very few problems with W10 running in "Desktop mode".
Touch on W10 on a VM only seems to work properly for the on screen keyboard. I can't start applications by touching the desktop or taskbar icons (works via mouse of course). W8.1 as a VM doesn't have problems with touch applications. My HOST OS fully supports the Touch features of the laptop I'm testing this on.
Not a mega deal but can't wait for the next release.