If you want to switch to ADSL2+ quickly (don't both unless your download speed is stable at 3.5Mb/s or better) then it may be necessary to request a manual migration to the WBC platform and upgrade to ADSL2+. The migration costs £11+VAT if you go via the manual process.
If you are not in a hurry you can wait for Enta to migrate the connection as part of their bulk migration process however Enta don't release any scheduling information so I wont be able to give you any indication as to how long you may have to wait for this. There is no charge for the migration if it's done via the bulk process.
Enta's WBC platform is good. The costs to them are lower on WBC and they are trying to move customer connections over as quickly as they can - BT place limits on the number of connections that can be moved over via the bulk process hence the delays - so Enta's bandwidth investment is primarily on the WBC platform.