When would be the right moment switching to Theo on express?

Right at the start.. no, actually, never. :)

I see Theo/Casher and Theo/Techer from the start finish top ten often...

Your only chance would be to do it straight from the start.

The way the market plays out, and the amount of turns on the server makes it not very ideal to ever convert.

I haven't played a Theocracy in a long time. They often win here though. The generally accepted wisdom seems to be what Kingme said, that a conversion doesn't work very well. But I seem to vaguely remember when I first started playing Express about 5 years ago that there were some sneaky tricks that people could play, dropping land and converting and all kinds of stuff. That was before this new GDI and right after the end of Earth 2025. I was a complete Noob and didn't understand any of it very well, but I liked the look of it and kept on playing and learning. Bu there is still plenty to learn, and there is the fun of the competition when you get pretty good.

I figured the switch would be pointless probably go Theo casher next set if that fails going back to the commie that won me 16th place a year ago. Thanks for the advice guys. I'm still learning lol

You would need ideal market conditions to benefit from the swap. If you could liquidate most of your tech and military into bushels to minimize the loss than maybe. But even that strat relies on selling the bushels quickly on the market after the change and the low D could also bite you in the ass(although this could be minimized by being in GDI and waiting for the last 6hrs where only people you targeted could hit so unless you expect an ROR/retal you are in the clear)

Rep casher...lol