If I apply for change of status, I .e from H4 to F1 and in the middle of the process if my H4 expires... what should I do??

You cannot transfer from one to the other if the first has expired. It would be a new application.

What does your Attorney say?

On re-reading I note the use of the word "if." You haven't applied for changed status?

If you don't have that 90 days window, then it's advised to leave US and apply for F1 at US consulate in your country.

You cannot transfer from one to the other if the first has expired. It would be a new application.

What does your Attorney say?

On re-reading I note the use of the word "if." You haven't applied for changed status?

No I haven't applied yet... nd even my H4 is n't expired... I still have 2 months...
So my doubt was what if F1 processing takes more then 2 months

I'm reading to allow 90 days - so, as was advised, leave the Country and reapply.