Hobbitmum,

You character has to transfer from Royal Scots Volunteer Light Dragoons to the Army abroad?
No problem. At this point there were no 'territorial' designations for most regiments and it was not uncommon to find recruiters from several different regiments turning up at hiring fairs and the like.
The Militia, Volunteer and 'Fencible units were also considered 'fair game' by recruiters and if your man was a 'trained' cavalryman<1>, he would be a prime target, especially if he was caught in the pre-1805 'invasion scare' (IIRC Rifleman Harris tells a similar tale of being recruited about the time of Corunna).
If a cavalry recruiter turned up at a RSVLD parade when emotion was running high, he might well leave with a recruit or two - regardless of where his regiment was 'supposed' to recruit<2>.

Regards

Walrus

<1>Levels of competence varied among some of the M,V & F units
<2> Providing the recruits could understand him, of course.