Yes, you have to enter the previous license key for any upgrade license key.
It is a pity that you cannot easily have LR3 + LR4 installed at the same time due to your win xp.
Because in case of problems you might want to be bale to go back, e.g. To export some parts of your LR3-catalog to use for trouble shooting.
Install the latest download of 4.1RC, not a boxed version of 4.0 as that for sure is too bug-ridden..
Maybe you are lucky and you do not need LR3, but be prepared to install it as well in case of problems.
If you install LR4 and have problems, you don't need to revert to LR3, just
set the process version to 2010. You will essentially be using LR3 then
and the performance issues should hi away.
You won't have all the fancy features of LR4, but you wouldn't anyway if
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Same here - no Lr4 problems whatsoever.
Good for you, but the advice to skip LR 4.0 and go straight to LR 4.1 (RC 2) is a good one.
Adobe themselves documented the serious issues with LR 4.0 that include faulty "Point Tone Curve Migration from LR 3 catalogs", broken "Edit In" functionality that also affects some plugins, and (for some) severe performance issues.