what exactly has happened? Did you send an invite and got no response?
I am in the same situation as you, and moved my children aged 8 and 6 from the much loved local primary to go private. Most people knew my oldest DS had trouble in his class concentrating(36) and did very poorly in SATS, was on IEPs etc. he aslo got beaten up a few times, which was the final straw for me.
I feel I have to tread carefully when people ask how they are getting on. TBH, I can only confess to parents of children who go to private school what an amazing difference it is. If parents of their old friends ask, I just say "yeah, it's early days, it seems o.k.".
I am VERY much aware that some of DC friends' parents are either anti private school and assume you must be a snob (much witnessed on MN as well), and other parents feel bad as they would like to move their DC but don't have the money. Few parents are neutral.
So I think it is really important to not wax lyrical, but to be a bit sensitive to this. I am also very careful that I do not dis the old school to my DC (kids talk!).
Also, I am fully aware that as I am now out of the daily loop, it is up to me to stay in touch with DC's old friends' parents/friends.
Only one "friend" has said; "Oh Chandon, are you sure you want to meet up? I thought you'd feel too good for us now". I told her to eff off and get that chip of her shoulder, which she took well 
Anyway, I think the effort has to come from you, it would be odd if ALL the mums dropped you TBH. So tell us how you invited them, and what they did or didn't do?