depends on what your previous service was
ie Army/Navy/ Air Force.
How long you've been out
How long you did
How long at substansive rank
if they have a slot you can go into with little or no re-training
Was in the Royal Navy for 12 years. Left last year (Oct) as a CPO (Ssgt). Looking to join REME TA. Can understand the massive jump involved and really wouldn't mind starting out again. Just asking more out of curiosity.
Here is a little ditty from me as I went through exactly the same situation.
I left the RN after 6 years in '07 as an OM1 passed in all respects for LET(WE) just waiting for a course. I joined the TA a month later and had to carry out all TA basic training and part 2 training. None of my previous qualifications have been recognised even though I joined the TA in a similar trade. My pay scale went back down to zero (except for my bounty which was top whack).
I would say join up because it is a good crack and considerably more professional and exciting than the RNR, but don't expect any special treatment apart from your bounty being at the top level.
Obviously your previous experience will put you in good stead during your career and that may be recognised by your seniors when they look at who to promote.
Edited to add: My unit where happy for me to be promoted to Lance Corporal straight away and resume my old trade, but the big scary people at Glasgow said no. Also you will have to relinquish your D2*** number for a sprog 300 service number.
I am now REME cap badged too. I was a small arms maintainer and worked WE part of ship, (close range, 4.5 and phalanx) for my whole career having done loads of engineering and weapons PJTs / Courses etc. Unfortunately none of this is recognised by Glasgow and I have to start from scratch completing a REME Armourer's Class 3 (the bottom level) course which is fun but a complete waste of time for me and the Army (not to mention cash).
Depending on your trade I suppose things could be slightly different, but I doubt it.
The Army are a good bunch on the whole but they have funny words for things like the heads and scran. I'd say go for it.