Just because your family is shifting a bit, that doesn't mean you should get lost in the shuffle. You still count. After all, a wedding celebration is one day. You've got the whole rest of your lives as an extended family to navigate together. So, it's important to set things off in the right tone.
A big part of this is making sure your own boundaries as a mother and mother-in-law are clear early on in the marriage, even during the wedding planning process. For example, maybe your idea of retirement is golfing, traveling and spending holidays with family, not babysitting five days a week. Or, maybe you assume (and hope) your son and his family will continue living in the same town as you forever. Seek clarity and check in now and then. The responses you get today may not hold true next month or 10 or 20 years from now.
You can be explicit without being awkward or confrontational. A good way to do this is by asking questions or stating what you look forward to happening in the future. However you share your expectations of family or future plans, make sure your message is understood.
Being a friend or family member is a reciprocal relationship. It doesn't have to be 50/50 all the time, but both people -- in this case, you and your future daughter-in-law -- must be willing to communicate and compromise to form a relationship. Otherwise, you're not doing any relating.
You'll always be there for your son. But what this support looks like moving forward now that he's engaged may change because his needs and priorities have adjusted. As your family evolves, celebrate the changes rather than resisting them. The journey will be much sweeter.