TELECOM 247

If I have 4 incoming BT lines all with the same phone number, and I want
to change them to 2 with a different number.
If I call BT and get them to split it into 2 services (2 lines on one
number then 2 lines on another), will I save 2 lots of installation fees
when compared to cancelling all 4 lines then getting 2 back with a new
number? (If that all makes any sense).

All lines are normal Analogue PSTN lines.