Migration from multimode to single mode fiber
You do not indicate what devices you are using at the moment, it would give a good idea of what serves your current needs.

Moving from multi to single is doable and often there are matching almost drop in optical devices that will have comparable specifications. 

More than 20km will need good design (noise issues in the RX circuitry) and infrastructure techniques (professional fibre terminations, not too hard to arrange these days). 

You also need to take the fixed delay between sites into account over those distances.

If your sites can see the sky and have some spare power budget it may be cheaper to provision good GPS disciplined local clocks as opposed to laying and maintaining many tens of kilometres of fibre cable.