Going from textarea to ajaxtext is moderately acceptable. We don't allow changing from frontend as it can cause serious problems in cases of going from text to drop-down. However in case of textarea to ajaxtext it should be ok. You'll need to edit the field within _comprofiler_fields and change its type from there.

thanks for reply, i tried just changing the field type, and didn't work... it was diferent but without possibilitie to save or insert new data on the fields using the ajax update, do i have to change more things?

You're performing an unsupported operation. I can't really help you much regarding this matter. As stated a direct database edit may not even work. You'd need to change its type. Edit the field in field management and re-save it so the ajax parameters can take hold.

other question is if i can change also other kinds of field to ajaxtext. like: predifined, emailaddress, webaddress and editorta!

No, ajax text field is just a normal Text field with ajax capability. It doesn't work for any other field types.