There might be an easier/official way, but this is how I did it:
(0. Close LR)
1. rename old photo folder to oldphotos or pretty much anything
2. Open LR. It won't find your photos and has exclamation marks next to them
3. Click the exclamation mark or right-click (can't remember) on one of the folders, there is an option to locate the files
4. Locate the new folder
LR should find most, if not all of your photos with just one click. It will certainly find all photos in each folder by just locating one photo. Depending on how complex your folder structure is you might need to do 3. and 4. a few times.
Also, double-check that your color labels are correct, I've had problems migrating them with earlier versions.