Upgrading to Sky Fibre & Phone Line - Downtime?

Currently have Sky ADSL on a second BT phone line that comes into the house that is used only for broadband.

I was looking at cancelling the second line and the ADSL and moving to someone (probably Plusnet) for rental and FTTC on the main voice line, however having spoken to Sky they've offered 12 months free line rental if I upgraded to Fibre Unlimited.

Either way I'm saving as it's £20 for a 38mbps connection vs. £31 or so (ADSL + rental) for a 9mbps ADSL connection.

Where I'm not 100% clear is on any potential downtime?

My understanding is that Sky do self-install on the 38mbps FTTC and I already have a Sky Hub 2 but I'm not clear what downtime, if any, is involved with a "double" move of both phone line and then the switch from ADSL to FTTC?

Phone side maybe 30 minutes and similar for the fibre side.