Either upgrading a wsp from 2007 or just simply copy-paste the folder to create a new feature from a starting point of another, will end up in solution and feature id errors such as:
(using my2010thingy as the name of the wsp as example)
A solution with the same name "my2010thingy.wsp" or id "xxx-x-x-x-xxx" already exists in the solution store.
my2010thingy.wsp: The Solution installation failed.
If the time lost creating everthing from scratch is unberable, try this:
1) open "my2010thingy.csproj" on the root folder
2) search for feature and package ids at the bottom and change them using the Visual Studio guid tool to trow registry-type guids (keep the old guids somewhere!)
3) go throw the folders in the solution, changing the old feature and solution(now package) ids for the new ones:
Now you may try repackaging, but I normally change the ids also in the existing package folders just to make sure I don't run into conflicts