Short answer, they are unlikely to be compatible. Symptoms may include flickering, poor low level dimming control or other issues of instability.
Old halogen lamps are seen as “resistive” loads to dimmers. Those lamps require “Leading Edge” dimmer technology.
LED lights use electronic drivers and require “Trailing Edge” dimmer technology.
If your old halogen lamp transformers are electronic, then an upgrade to LED “may” work with the existing dimmer.
Forcing incompatible technologies together may result in damage to the dimmer or the driver components. That damage can, under some circumstances, take months to manifest.
However, rather than risk a longer term issue, upgrading the dimmer to the latest LED compatible technology is a safer bet than can bring other benefits. Some of the latest dimmers have additional features built in, such as timing functions, delays, in-built switching functions and control knob/button illumination to aid night-time visibility.
Yes, the cost of the LED upgrade will be marginally higher, but the dimmer will remain a small portion of the total upgrade cost and the smooth dimming and peace of mind….priceless.