My inlaws moved to one a year and a half ago.....there are pros and cons as I see it.
Firstly after they moved their FIL broke his hip, and never really got back to being well before he passed away. If this had happened in their previous home it would have been a nightmare. As it was he had care available, a much more accessible home to get about in, the work of downsizing etc had been done before they were too frail to participate etc. From that pov it was incredibly good.
However, their daughter did 90% of the work in selling/downsizing/ moving them, my DH (the son) didn't do much of the leg work as he had other things to contend with, so all the packing, arranging etc fell to her, and then MIL blamed her for moving them in a very unfair and unkind way. The upshot of this is that their relationship has now completely broken down and SIL is understandably incredibly hurt by this.
For this reason I would tread very carefully about moving them if they aren't both completely on board or you might well end up being the scapegoat too, however sensible it is to move practically.
Finally, financially these places are a nightmare. The worst property investment by far in the UK. Do your parents realise this and would they be happy for their estate to take that kind of financial hit in order to live there? (I was aware they have very high exit fees, when you sell one, but was not aware how massively poor the resale value of these properties are before the exit fees are even considered, until there was a programme on about it on radio 4 last week, I think it was on You and Yours, I tried to google it for you but couldn't find the programme, but did come across this article in doing so. Food for thought, and I'm not sure my PIL would have gone for it had they been aware of this factor.
The thing I have come to understand is some people have problems with mobility etc when they get older that need to be addressed, but some people have complex emotional and mental issues (even before they get old and frail) and this makes dealing with the problems of old age much, much harder for the family. It's not easy 