Thank you to Alm (and Håkan on the Tek forum) for helpful information as well as Dennis Tillman, W7pf for his help.
To close the loop on this question, yes it's entirely possible and not particularly difficult to upgrade a TDS20xxC series.
It takes three pieces/resources/steps.
First is to install the communication software (BOTH packages are required) from Tek, including Openchoice Desktop and TEKVISA.
Then establish communication with the scope (USB cable) via the software and change the model. First, I read the identity of the scope using the "*IDN?" command via QUERY. Second I sent the PASSWord command and the password INTEKRITY via WRITE (not QUERY). Older models used PITBULL, but newer models (like the TDS20xxC models) use the newer password. Then I sent the MODEL command and the new model number "TDS2022C" via WRITE (not QUERY). Lastly I read the identity of the scope using the "*IDN?" command via QUERY again to see that the model had been changed.
The third step is to run a full factory calibration via the "Adjustment Procedure" instructions via the Service Manual. If the calibration procedure completes successfully, it will write everything back to storage and the model number change will persist past the power-down of the scope. The Self Calibration will NOT write the model number change to storage and it will be lost after power-down.
It's not hard to do, but it takes certain resources. The calibration is the most tedious step and takes a stable power supply that can be adjusted down to 0.015VDC, a leveled sine wave generator, and a square wave generator, as well as a couple of identical coax jumpers, a T and a 50 ohm passthrough terminator. Check the service manual for the specifics. With all the messing around, it took around 2-2.5 hours to get this all done.
Cheers,
Grant