What is called cross-talk is what is probably occurring to your ADSL This is where signals from one set of wires creates interference in another set of wires. ADSL suffers from this but FTTN & FTTC are sort of immune due the technology used to provide these services. Fibre Optic does not have this issue at all.
Once they pull ADSL out from your area FTTN speeds should increase a bit because it cuts some of the cross-talk/interference caused by the ADSL co-existing with the FTTN. They reduce FTTN speeds until then to help reduce the interference ADSL users suffer.
Could your outcome be worse when you switch to the NBN? Very unlikely but it might not be staggeringly better. VDSL (what FTTN uses) is faster than ADSL both up and down until the copper distance becomes quite long or is very degraded and then both are about even. For an older (2015) and basic read about the FTTN VDSL deployment and why VDSL cuts interference see this NBN Co article (was created/posted before the big FTTN push so makes reference to things now in place):
Also read this topic created by @steffie5904 about their change over to the NBN (it is a long thread but many points could be useful to you):
I also would advise to think about (weigh up) an earlier change over to NBN rather than delaying until ADSL cut-off. This may cut changeover delays and issues substantially.