Exeggutor TM?

Yes if you're unable to do it with Extrasensory, as bullet seed is the best move on an Exeggutor when it's double super effective.

It's also better than Extrasensory when only single effective but not by much.

The downside is that they'll no longer be useful for soloing Machamps, which is why I suggest TMing only if you -need- Bullet Seed to be able to solo Omastar.

I changed my highest Extrasensory to Bullet Seed because I wanted double-Grass on at least one of my Eggies. I also already had C/SB, so rarely pulled out the other one. Now I find myself going to it much more often.

Yes, TM them. With ES, they are not that great against either Machamp or Omastar. With BS, they are the best grass attacker in the game. There is a huge difference in effectiveness against Omastar, for which you need at least five good double grass attackers. I failed with four plus two powered up legendary electrics, and then I swapped out the electrics for two CP 1900 Victreebels, and beat the Omastar with 20 seconds remaining. It's critical to fully exploit that double weakness to grass.

ES/SoB Eggy is a good solid generalist, but with the current gym system, you don't need an army of generalists, and Dragonite and T-tar do the job better. When it comes to raids, DPS is critical, so the compromises that make good generalists work against you. And it's not like you'll never be able to get ES/SoB again (if they had confusion, I might hang onto them).