Yup it does change as you use up fuel, but that's normal and accepted.

Most Ultimates ( and for that matter most planes ) have the same setup.

Heck my Funtana 90 which hovers with ease, has the tank practically in the cowl!

The C.G. is calculated for a "worst case" ( empty tank ) situation.

By doing so there is no risk of configuring your plane in such a manner that it will be unflyable.

I've hovered mine at altitude, never close to the ground, and I really do not notice the c.g. variation.

That said with close to 1/2 an hour of flight time until the tank runs dry, I never run the tank down to more than 1/2 ( about 15+ minutes or so ) so that there is little risk of fuel starvation on downlines when the clunk gets uncovered.

( and YES the clunk gets uncovered constantly in downlines for those who doubt it, proven on film on other threads... )

My C.G. is about 1/2" further BACK than recommended... because of the weight of my smoke system and battery.