Thanks for all the answers:
tubaboy wrote: Do the trees have any new growth? Sometimes they respond to changes pretty drammatically but many leaves grow back... that's what happened to mine when it was an "indoor elm"
Yes in the biggest one i see new buds, in the smaller one not really. It's my first repot for elm's and i just don't know if this a "normal" behavior. It seems strange that only after two weeks it started to turn yellow.
tubaboy wrote: Dormancy would mean that the leaves fell off and it stopped growing. This doesn't look like that happened. Where are you? What kind of seasons do you have? Reoptting is usually only a good idea at the right time of year and when the tree needs it..
I said that "i think" it's dormant because there was any grow or change in November, December and beginning of January. I am on North Hemisphere more properly Portugal, Lisbon.. The lowest temperature we have this winter was 2ºC so this is a very mild climate.
Both trees definitely needs the repot (mainly the smaller one that has a very small pot to it's size), and i think in our case the proper time to do the repot is end of January. I read some info like this: "In late January after the frost, before starting sprouting always with a strong pruning, or in summer with total defoliation."
Link to Info (Bonsai Store in Portugal)
Bonsai Bender wrote: I would have to agree with Tubaboy. That does not look dormant. It is best to wait until Early spring (when the buds are swelling or extending) to repot and do any root work. If you are in the Southern Hemisphere and are in the middle of summer you will probably be okay. Just keep it out of direct sunlight until you see new growth. Don't freak out and drown it either.
Answered above.
It's located on half sun spot now, don't know if it's important but it rains all week before the yellow leaves appear.
m5eaygeoff wrote: One of the trees looks very dry. How are you watering? It is quite normal for this species to lose leaves it after all a deciduous species.
If you re potted properly then it should be ok
It was watered right after the picture

the other one was ok. Yes it's normal, but not suddenly as this. Also this has to be related with repotting because both repotted Elm's has the same behavior.
Thank you once more for the answers! I will update with pictures