The main thing will be to concentrate on the SOUND... Obviously, the BK5 being pretty much the BOTL of Roland's main arrangers, and the VA76 having been the TOTL (and over $2000 more!) you aren't going to get the quality of build or keybed that you got before.
But, especially if you jack it into a decent sound system (the VA didn't have speakers at all, did it?) and perhaps take off the Mastering Tools (the Master EQ and Compressor), you might be pleasantly surprised at how close they come. Personally, I am never a fan of the Mastering Tools. They always seem to come with factory defaults that are designed for poor speakers, and over-hype the sound with too much compression and EQ for my tastes. If I remember rightly, the VA76 didn't have these (I may be wrong, though), so if you are looking for the best A/B comparison, listening with AND without them might be a good idea.
Fran, who has one, will definitely be able to help more than me (I have a BK-7m) but we will try to step you through it all. Plus, of course, the Roland-Arranger.com website has a lot of resources and advice.
One last thought... Do you still have the styles that came in your VA76? They should play pretty well in the BK-5, and familiarity might help you reconnect with your fond memories of the VA.
If I were you, after getting the basics of barebones operation of the BK down, I would pay special attention to the manual's chapters on the Makeup Tools. This is the area where you can quickly change a style's sounds, volumes, effects, etc., and mess with the drumkit (tuning, sounds, velocities, etc.). It is BY FAR the fastest, easiest way to tweak a style to your liking of any arranger out there, and is probably the feature you will find yourself using on a daily basis (at least if you are a tinkerer with your styles, like I am!). It can quickly turn a style that doesn't float your boat into one you love with the minimum of effort.
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An arranger is just a tool. What matters is what you build with it..!