The upgrade uses ADSL2+ tech at the exchange, which is 21CN
Uh uh

!
You have that the wrong way round. 21CN is ADSL2+.
ADSL2+ is a higher performance version of ADSL, using twice as many downstream frequencies. Most LLU has used ADSL2+ for years - I believe some early TalkTalk exchanges only gave 8Mbps LLU, but I don't know if that was ADSL or ADSL2+.
BT's 21CN broadband is just BT Wholesale putting their ADSL2+ kit into the exchange,
to catch up with the LLU providers. 21CN does not affect LLU in the slightest.
My broadband basic info/help site - www.robertos.me.uk
Domains,website and mail hosting - Tsohost.
Connection - Plusnet Extra Fibre (FTTC). Sync ~ 56.0/13.9Mbps @ 600m.
"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.
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Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.