The earlier you speak to the venue, the more likely you can change the date. Just explain, and see if they will move it. If you've only just booked it, it'll be more than fine, they'll be used to registrars not being available and people wanting certain months.
7 months to live with someone is a good amount of time to decide if you get on living together. But depending on your wedding plans, you'll have more and more deposits down and it will be harder to call off if you feel that way.
On one hand, I'd like to think, after 2.5 years you should be fine, there will of course be little niggles that you could resolve. I'd get ahead of this now, discuss anything that might come up, even if it feels silly. Like absolute darkness, or sleeping with th light on. Music or silence. Bedtime, for you and the kids.
On the other, Weddings are a pain enough to arrange without throwing in moving, living together for the first time, and sorting and selling his late mums estate, plus allowing time to grieve.
Between all of them, I can see the wedding planning not really being a focus until maybe May..., 6/7 months is not a lot of time depending on the size of your event. Anything is doable, but if you wanted wedding fairs and time to look at all the options, it's going to go quick!
Personally I'd change the date, but not through concerns about living together, more that there's so much happening in such a short time. But then, I like taking time to bask/savour/chill/settle in to what I've completed. It took 18 months after buying our house before we could be arsed to start wedding planning again.