Can I merge my old Apple ID to my new Apple id

No. but you can move data. This article can be useful for that:

Archive or make copies of the information you store in iCloud – Apple Support

And you can use family sharing to share some of the purchases. You need to have the old apple ID in a family sharing group.

Share App Store, iTunes Store, and Apple Books purchases with Family Sharing - Apple Support

My advice would be to keep using your old apple ID. You can change the name of it instead.

Change your Apple ID - Apple Support

Good luck!

Not normally. You can check using the steps in the article below.

Serial number – Find of your Apple product

I had an Iphone 6 Plus, lost it and bought an Iphone 8 Plus. All data was recovered from the cloud. Is the serial number the same on both phones?

Maybe I miscommunicated. I lost my 6 in 2018, bought a new iphone 8 Plus to replace it. Now I need the serial number from the 6 Plus Iphone. Is that recoverable?

I had an Iphone 6 Plus, lost it and bought an Iphone 8 Plus. All data was recovered from the cloud. Is the serial number the same on both phones?

No. A serial number is unique per device, they are not the same