While it is true that you can't have more than one Regular Saver, it doesn't mean the offer is closed to you. In fact, you are well within your rights to close your existing Regular Saver and open a new one at the 10% rate. The question becomes whether the interest penalty you get on the early close is made up for by the bigger rate. This all depends on your maturity date.
Also, if you are an existing Regular Saver and your account rolls over during the offer period, you will automatically get the 10% rate if you continue to save with payment into the account being uninterrupted. Ask anyone from Halifax, they will tell you the same!
Also, there is an offer on the Guaranteed Saver which offers a reward rate of 6.25% fixed. There is limited access though but could suit some looking to earn a higher rate and get the extra 2%.