I knew it was going to happen!
In ArcMap 10.5.1, once labels converted to Annotation Feature Class in a FGDB, it is posssible to change the reference scale in the Properties. It is an unusual function as alignement might appear... However, very handy in certain situation.
In ArcGIS Pro 2.1, I made a map at scale 1:900,000 and edited all annotations. Unfortunately, I just discovered that my map fit my page at scale 1:1,000,000 !!! Is there any way to change the reference scale of an Annotation Feature Class stored in FGDB? Or should I wait for Pro 2.2?