Hello and oh boy do I understand! 12 years ago I was sent a lilac twig in the mail. A family member had died and this was a memorial to her but I was in no state to deal with it so I just put it outside in a flower bed to deal wit it later. Now it is hitting my garage roof line. Lol!
I don't think I even could move it now, I'd have to dig up a small one, establish it somewhere, and somehow figure out how to kill the existing one. They multiply via the roots so you may have more growing than just the bucket one.
I don't know for sure, but lilacs are a hardy bunch. I bet you could cut the bucket free and just plant the tree where you want. Water it really really well for 24 hours first, cut, replant right away, keep watered and give it root grow fertilizer for a few weeks, then watch really close to make sure it is watered well the rest of the summer. The alternative would be to cut the pot off around it and dig out the roots.
When I moved a tree, I was told to trim the branches because the roots would be getting severely cut down and it wouldn't be able to support all the branches, so I needed to balance it out by pruning the branches down too. I'm not sure if lilacs are the same way. They're pretty hard to kill though.
