You said you were using some of the grape concentrate to top up meads made a year ago. Couldn't this restart fermentation of perhaps change the flavor profile of the meads?
So, sorry mate, I didn't see it, and only spotted it today..... my bad!
In response to your question, Yes, it's entirely possible that using a flavouring like a grape concentrate could change the flavour profile. Though if you think about it, it depends entirely on the flavour of the top up liquid. This is an idea from Keith. He originally suggested the use of Chenin Blanc grape concentrate as it has distinct "honey like" qualities. It cost me nearly £70 to learn that he was absolutely spot on (the only way of getting Chenin Blanc easily here, it to get a kit and use the "juice" from that).
Sure it's grape juice/concentrate, but when I took a little taste it was very like tasting watered down honey. Of course, there's a few "pro's and con's" to everything. The pro's, to me, are that you're putting something in that tastes not unsimilar to how you'd expect a mead to taste. Also, if it's concentrate, it will add a little more body/mouth feel. There's also the possibility, that depending on what it is that you're topping up with, that you might also lighten/darken the colour as well.
Ok, so I think that makes sense.
The con/con's ? Well yes, it's possible that it could indeed restart a fermentation. Though if you've finished the fermentation stage and done the usual "sorbate/sulphite" stage as well, that should, pretty much, prevent that from happening. Also, if you've filtered the mead, that can (depending on the filter size/gauge) remove some yeast cells. I personally am happy to filter, but not using a "sterile"/ultra fine element - of the 0.25 micron size. I like to use either 1 or 0.5 micron sized elements - so I try to make sure that I don't forget the sorbate/sulphite stage.
A point in fact, is how much you're actually adding, as to the effect it will have on flavour. If, for example, you're in the habit of racking until you start to get some of the lees through the racking cane/syphon - then the last bit of liquid is poured off, into a tall, thin container, then refrigerate that. You'll find that in a couple of days, you can easily remove more clear, or relatively so, wine/mead. Thereby minimising any racking losses. For each gallon, you're gonna lose what, 1/4 of a pint, probably not even that. So if the flavour of the top up is even vaguely close, it's unlikely to affect the flavour of the wine/mead too much.
You might actually be doing something to deliberately change the flavour, like using brandy or whiskey to top it up, that will change the taste. It's also "fortifying" the wine/mead, so might then need time for the "new" flavour to develop.....
Of course, there's always gonna be exceptions to the rule, and in this case, you might intentionally want to restart fermentation, to produce a sparkling mead. So that being the case, then I'd just have cleared/racked it in the usual way, leaving out the sulphite/sorbate stage and then added the grape juice/concentrate.
I understand that Keith is a great believer, in just making a basic mead, getting the fermentation going and then working out how to add any other flavours you might want.
That might entail things like after primary fermentation has started and you've then moved to secondary (one of the good reasons for using a bucket for secondary ferment) then you want it to have a fruit flavour, so you can either ferment on the pulp and by that I mean add the fruit in a muslin/cheese cloth type bag and then putting it in the secondary ferment. Or just crushing/squashing the fruit and putting it in directly - if it's the sort of fruit that just goes to mush, I try to use a bag, otherwise you can easily end up with it being a complete PITA to rack off the lees/fruit pulp. Or you can extract the fruit juice in some way and add it at the same time.
I like to do that with my steam extractor for all fruit, except the ones that give a "cooked" taste if heat treated (the steam extractor is brilliant for "red" fruits - Apples and kiwi fruit are two examples that you can't used steam on, they both give a pronounced "cooked" flavour if you do).
I also like to hold some of the fruit flavour back, as I've learned and found that if it's added after the ferment has finished, you get more of a fruity taste that doesn't get diminished/reduced by the action of the yeast doing it's ferment thing - it can also help with making the mead ready to drink quicker........sugar/sweetness, can often cover "a multitude of sins"......
Hopefully that gives you a clear enough answer to the point I was trying (badly) to make.......
p.s. and sorry if that answer seems a bit "disjointed" I was trying to make it all clear but kept having other ideas/suggestions - so made extensive using of "cut 'n paste"....