Why was the change made
Did you have a lawyer acting for you both in the purchase?
If so, why did the lawyer make this change without first confirming your agreement to it? or did they have permission to take instructions from your husband on behalf of you both?
Have you asked your husband why he did it?
What does the change mean
Joint tenants - means whoever survives automatically inherits the Property... regardless of what the Wills says.
Tenants in Common - means that your share will go to whoever it is left to in your Will.... therefore, you need to ask your husband if he has a Will, and if you can see a copy of it, to determine who his 9/10 share has been left to..
No impact on Separation/Divorce
With regard to separation, it does not change you entitlement to a Property Settlement, if and when you separate.