For Capital Gains tax, it is your choice. You should write to HMRC saying when it became your main home to avoid later argument. If you are married, your spouse cannot have a different main home to you. Your previous home ceases to be your main home, so will start clocking up Capital Gains. You may find it an advantage to have, if not a valuation, at least examples of homes in the same area and what they have recently sold for to support your claims of current value.
For Council tax, start by notifying the two councils concerned. in some cases (e.g. areas blighted by holiday homes) they may require further evidence that you really are living there most of the time.
For other purposes (planning permission etc, esp in areas blighted by second homes) the council may require evidence, and may refuse to re-classify it. Start by writing to them.