Your assessment is nearly correct but not quite.
It can be done, but not the sort of thing newly-trained Tier 1 support at Plusnet would be able to handle. Some of the more experienced ones, or higher tiers should.
If you migrate the broadband away Sky will let you keep the line rental with them and change it to a normal "BT line". (Wholesale Line Rental which is what all non-LLU lines are). AIUI Sky would need to provide a MAC for the broadband but agree I am correct.
As has been said, your Plusnet costs would be higher. Both the broadband monthly charge and a setup charge that isn't levied if you take Plusnet line rental. If you do take Plusnet line rental you will normally be able to keep the same number. Was it originally a "BT line", or was it installed by Sky from scratch?
My broadband basic info/help site - www.robertos.me.uk
| Domains,site and mail hosting - Tsohost
Connection - Plusnet
UnLim Fibre (FTTC). Sync ~ 56.6/14.1Mbps @ 600m. - BQM
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