I agree with Uberdice. I would have taken longer, so there's that as well.
If you want to switch fully trained crews into a new tank, you definitely SHOULD have them trained to 100% in their crew qualification, especially if you're not using Gold to retrain without losing XP. I'm a cheap bastid and retrain with credits MOST TIMES. On occasion I will use Gold, because I had that crew for so long and I really don't want to lose anything from them for the new unit.
Overall, as I have played lower tiers mostly (nearly exclusively due to my extremely potatoey craptop that gets 11 fps at lowest SD quality settings on a rural wireless connection) I have a bit of familiarity with crew switching when they are not fully trained as well. And, because I am a cheap bastid and a FTPP, I use credits for most of my transactions with crew, I know a bit about the math of whether or not you should retrain a crew to a new tank or just get a whole new crew.
This is different for Premium Crews that come with Premium Tanks (I call them premium Crews, because those are the ones that tend to be Zero Skill BIA crews that come with a Premium Purchase or an Earned/Won/Gifted Premium unit). You SHOULD take these crews out of those Premium Tanks and slap them into something more fitting -- and, in this sort of instance, I would spend GOLD to do it, so you don't lose any XP value on your Skills/Perks during the transfer.
Any other crew, I most likely will take the Regimental Training at 20K credits per crew when I move them to a new tank. This is ONLY when I switch from Same Class to Same Class. Like a Med Tank to another Med Tank. That's a 10% hit to your overall XP as long as your crew is above 85% Qualification, it is worth moving them for credits, though just barely.
In that Crew section I linked, you can see that a Regimental Training level of Retraining Crews provides you a 75% minimum Crew OR 90% of their Previous Qualification. In short it bones you for 10% of each: Crew Qualification (your ability to operate your tank) and Each Skill or Perk in their File. Thus, if you have a 100% crew, with BIA, another skill full and part of a third, it will take 10% from EACH of those.
It is hard to note, because it does it like this. Example: 100% Crew, BIA, Repairs, Firefighting (I use these for purposes of ease of example as ALL crew can have these particular skills/perks) and the last one is at 70%.
Crew BIA Repairs Firefighting
100% 100% 100% 70%
Train to new tank with Regimental Training @ 20K credits per CM. Same Class of tank, Med to Med. 90% of original value. What it does...
Crew BIA Repairs Firefighting
90% 90% 90% 60%
How it Looks when you see it:
Crew BIA Repairs Firefighting
100% 100% 100% 30%
The reason it looks like this, even though it is actually taking 10% from each of the skills and crew qualification is because the system is automatically reapportioning those XP points to ensure that all the Qualification, first and second skill slots are FULL (at 100%) so that it can start adding more points to the last Skill/Perk in line. This is automatic and the reason is that it makes it easier for the system to continue training your crew in a stable manner. After all, you wouldn't want to have to wait for your Crew Qualification to reach 100% from 90%, then start working on your BIA until it's done, then start working on your Repairs, etc. until they are also done.
That would be too painful and hard on the players. So they scoot the extra points from the last skill or perk and use what it has to 'fill' the balance missing from the ones before them as much as it can until it's at zero. This is why some folks say, "Don't retrain or reset Crew Skills until you have your last Skill/Perk at least at 50% (for normal two or three skill crews being moved)" This is to account for that automatic reapportionment such that it does not leave you a zero % Skill to start over from scratch.
If you have a situation like that and feel you just HAVE to keep the crew and move it to a new unit, then, if you have it, I would use GOLD to retrain. I wouldn't do this as a 'casual' affair in my case. I am a cheap bastid, because I am. I don't mind spending real money for things of estimable value, I just don't have it to spare in RL for this game at this time, so I have to be cheap and patient. I mean, why else would I be using an aging laptop made in 2010 to play if I had cash to spare, right?
So my advice, when it comes to 'currency transactions' in this game is to use credits whenever possible, take the hit to the %s and go on from there. If you have gold and plenty of money for a premium account, by all means, please use gold and help keep the game funded. You support me when you do that, and I feel that's a good thing. 
In return for this beneficence by paying players, I have accepted my role as cannon fodder for the paying players and have also taken it upon myself to do as much as I can to 'give back' to the community that supports my playing. I hope that it equates to a fair deal all the way 'round.
GL, HF & HSYBF!
OvO
Edited by dunniteowl, Aug 28 2019 - 16:09.