does anyone know if i change an existing horse 4yo for the 3 yo version is this allowed also presumably you can only have one version of the horse running just a bit of clarification please

No. You can't change ages during a season. Ages must also be usued sequencially. If you currently run the 4yo in the league, then next year it must be used as a 5yo (if you use it next season).

ok that makes life easier i can loose a few horse from my files now got way too many