What do you hope to gain b y going to 26" wheels over 700c wheels with similar tread tires?

Honestly, seems like a big waste of money. The 720 is just a typical flat bar hybrid, basically an 820 with canti posts up higher to accommodate 700c rims.

I would just source a 820 frame/fork and call it good. It would not be worth it at all to modify and move the canti posts on the 720 frame.

But again, I ask you, what do you hope to gain?

Seems like a huge waste of time and money when older nineties era mountain bikes are all over craigslist. You will spend hundreds of dollars geting the posts cut off and rewelded and the bike repainted and in the end, you will end up with nothing better than a typical 820 mountain bike.

Since you have the donor 820, just move the entire drivetrain of the 720 over to the 820 if you so desire. Then you will get your 26" wheels.

In the end, you will have spent a huge amount of money to make a bastarded 720 that can't use the desirable 700c tires and will be worth less than a stock nice 720 multitrack which in itself is only worth about 100 bux.