name change question

I live in PA, am in my 40's and getting remarried. I plan
on keeping my current name at work and using my new
name socially. Can I legally use both names interchangeably?
I have property, stocks, etc and so many accounts accumulated
over the years that it'd take forever to change all of them.If
I use Mary Smith Brown as my new legal name, is it still
legal to have an account and sign as Mary Smith?
thanks!

Yes, you are allowed to have one name for legal documents and another to use socially.

Why you would want to do that is a mystery, but it's legally permissible to do so, unless you're doing it for some fraudulent purpose, such as dodging creditors.