The ISA rules have changed as of 6 April 2024. You can now have - and contribute - to as many ISAs as you want (except for Lifetime ISAs and Junior ISAs). Provided you don't contribute more than your £20,000 ISA allowance across them all.
If you want to transfer your Stocks and Shares ISA to a new provider you can still do this. If you transfer, you can continue to contribute to both your existing ISA and the new ISA that you have transferred to.
Learn more about transferring to Stocks and Shares ISA