Apple watch
I bought the non cellular watch. If I eventually want cellular can the watch be activated as one or would I need a completely new watch with cellular
The GPS only Apple Watch does Not have a cellular radio chip.
If you want to get cellular service, you will need to buy a Cellular Apple Watch.
You will also need to have an iPhone 6s, SE (1 or 2), 7 or newer running iOS 13 or newer (assuming you are talking the Series 3 or 5; all bets are off after Apple Releases the next Apple Watch version as the minimum iPhone may change)
The Cellular Apple Watch shares the iPhone's phone number. This requires the iPhone cellular carrier to support the Apple Watch. The iPhone carrier charges a monthly fee for this service. In the U.S. the fee is $10 per month added to your iPhone bill.
iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service: