What does the guardromon in factorial town who says he does remoddeling..

By remodeling he means that you need to bring a data type Digimon to him so he can change. Also, he is not there during the night time. Myotismon joins the arena when you complete Grey Lord's Mansion the first time and SkullGreymon join when you beat it the second time. You need to find him in the mansion when he says he wants to retire - or after you complete Mt. Infinity and go back there when it has become Back Dimension you will find him.

I remodelled my mamemon and it tranformed into metal mamemon by charging 2000 bits

Guardromon can be paid 2000 bits to upgrade digimon. Mamemon is definitely known to upgrade to MetalMamemon, would need testing to see if others would work eg, Greymon, to MetalGreymon, test yourself if need be.

Myotismon can be found in his secret lair below his bed the first time, the second time he can be found only after reading the newspaper article. I can't quite remember how to do it, BUT I think you have to go below his bed again to see he's not there then go to the main room where you enter then the first door on the right where a Tsukaimon stands is like a little closet. This is an elevator, which should be available, go down it and then go through then basement level and you should find Myotismon, go prepared as you must fight SkullGreymon now and there may be other monsters there..

The Guardromon can be used to remodel Mamemon (and ONLY Mamemon). It has three possibilities: "It was successful!", which will immediately digivolve Mamemon into MetalMamemon. Next is "It didn't work.". This does nothing, and is the worst result. The third result is "It didn't work, but...", which will immediately digivolve Mamemon into Giromon.