There are seats that are harnessed then move to booster mode - but there are dedicated seats that are better in both categories and the way things move in 2/3 years when your 20month old is moving out of a car seat with 5 point restraint I think there will be better seats on the market - so I'd look for a now seat.
My 115cm - 4year/9month old is still in her Safe & Sound Tilt & Adjust seat - we will need to look at something else for her soon (this seat rear faces to 12kg - she was in it til 18months that way)
Maxi Cosi Complete Air is another seat worth having a look at.
The Safe & Sound Maxi Riders do both 5 point harness & then onto booster mode - they rate better as harnessed seats though.
HTH a little.