If I upgrade to 2019 using an upgrade serial from X7, do I need to include the previous serial that I used which was X5?
I don't think so.......
I'd test 2019 to see if it suits you before you set all your files in 2019 stone.
Wait till Update 3 at least. 2019 isn't really ready for prime time yet. I don't think it will ask you for a serial number in X7 is installed on your computer. If you have removed X7 (I wouldn't just yet) it may ask for it. Or it may ask you to sign in, then it will check with serial numbers already registered with Corel.com
Wait for update 3
Will do. When is update 3 expected?