You don't have to pay an additional months fee, but you do need to get your timing right. ISPs are "entitled" to their terms and conditions, 30 days notice seems a long period, but its in the Ts & Cs which you agree to when you sign up.

When I recently changed from Voda to Slingshot, signed up with Slingshot, got a confirmed changeover date which was near the end of my 30 days with Voda. Slingshot initially wanted to do the switchover straight away, but I pushed back on the date and got it programmed towards the end of the cycle. Once the date was confirmed, then advised Voda I was cancelling my account, and the effective date of the cancellation being the 30 days (right at the end of the billing period). I did cut things tight, switch off date with voda around the 1st of the month, active date with Slingshot around the 29th (so 2 days early).

All went without a hitch. Not sure if I was just lucky or because it was a simple transfer of an existing fibre connection from one ISP to another, that the process of the change itself is straight forward, but I was getting emails / texts from Enable confirming changeover date, a warning a few days before changeover date, a text / email on the day it was due and the same again when it had all happened.

Unplug old router, plug in new one and aside from a quick call to Slingshot for a slow uptake on the connection, all good.