After I upgrade, can I continue to use the old version, or sell it to someone else?

No. That is a violation of our software license agreement and is strictly prohibited. You were sold one license which permits you to use a single copy of the software on a single personal computer. That includes any and all updates, no matter whether the update was minor (e.g. 4.00 to 4.01) or major (e.g. 4.12 to 5.00).