Well, if it really is "no big deal," then why celebrate it? Make the announcement as simply as clearly as possible a couple times before the move. I’d put a nice reminder in all bills for the three months prior to the move, and a reminder after the fact. A personal email from attorneys to all their current clients would be good, too.
If you do want to make a marketing opportunity out of it, focus on the reasons for the move, rather than the move itself. Are you growing? Need more room? Make sure you provide introductions to the new lawyers, practice/industry groups, etc. and thank clients for their participation. If you’re only moving because it’s a cheaper space… well, that’s harder to market.
If you just need an excuse for a party, make sure you put it that way. I never minded being invited to openings like this, so long as I wasn’t sold a bill of goods that didn’t live up to the hype. I went to the "grand opening" of a friend’s new store a few years ago. The invite talked about a "brand new store" and "the future of the business." I got there, and the store looked identical to her old location on the inside. No new format, no new merchandising, no new products. She moved because it was a better lease and more convenient for her. How is that important or memorable to me?
Another reccomendation I make is to invite non-clients who were involved with the move, including your new neighbors. If you had facilities help from outside consultants (decorators, contractors, painters, etc.) invite those who did a great job and whose work you’re proud of. They’ll love to mingle with your clients and "show off" their latest work. Sometimes the best way to network is to help others network.
Stumble it!