There are two ways to turn a 240V circuit into a 120V circuit:
Use one of the previously-hot wires as hot and use the other previously-hot wire as neutral, or if you already have a separate (3rd) wire for neutral, use one of the hot wires plus neutral. Super easy but you get only get to use 1/2 the circuit - i.e., 1/2 the power.
Multi-Wire Branch Circuit
An MWBC uses 3 wires - 2 hots + neutral - to provide 2 full 120V circuits. This can work provided the hots are on separate poles with common shutoff (which you already have) and you have a 3rd wire for neutral (see below). It saves a wire compared to two separate 120V circuits because the neutral only carries the difference between the current in the two hots instead of adding them together.
I'm not going to try and explain the details of MWBC. Search elsewhere in DIY SE and you can find plenty of information. But a few things to watch out for are AFCI and GFCI - now that you are redoing circuits, one or the other is likely to become a requirement and you can do that in the panel (easy) or elsewhere (gets complicated with MWBC).
The key question is whether you have neutral in the cables or not. 240V heaters do not necessarily need neutral. By comparison, clothes dryers and ovens typically do require neutral in order to provide 120V to controls & lights.
The problem is that neutral must be white or gray. There are certain exceptions (due to the way standard cables are produced) where a white wire can be used as a hot or switched hot but not, as far as I know, the other way around.
There are 4 possibilities:
Wires in conduit
If you actually have wires in conduit instead of cables then this is super simple - just add a white neutral wire (if you don't already have one) and then wire up an MWBC.
You can't add a 3rd wire (you would have to replace the entire cable), so move the white wire at the breaker to the neutral bar, replace the double breaker with a single breaker (you get the other space back for a new circuit) and you have a standard 120V circuit.
Black & Red on the two poles of the breaker (as originally installed), white on neutral bar and you have the perfect setup for an MWBC.
Some other cable
If you have two non-white color wires in a cable (e.g., black/red) and no white wire then I believe you are stuck. You can't turn a colored wire into neutral and you can't add another wire without replacing the entire cable. You could use the cable to power many LED lights (they typically have a wide voltage range) but not ordinary 120V receptacles.
If your existing circuits have ground (either a ground wire or metal conduit) then you are all set. If not then you may or may not be able to reuse the existing wire without retrofitting a ground (basically connecting a ground wire from the new receptacles to some existing ground elsewhere in the building).