I'm a bit confused after hearing that roses emit something into the soil to
prevent other roses growing in the same spot.

It is a soil pathogen thing not the rose itself. A bit like holiday
bugs for humans. The locals never have a problem it's just the newcomers.

I want to replace some ancient roses, am I going to have problems?

Take out the old rose which will probably be the easier part of the
exercise. Next, if you can, dig out a two spades widths cube of soil and
replace with soil from another part of the garden then plant the new rose.
If the soil replacement is either too much work or just plain old
impractical then nip down to the garden centre and buy some Armillatox. Mix
this up according to the instructions and give the soil a good drenching
with this. Wait for three days and plant your new rose. I have used this
method and had no problems.
Lastly remember to water a newly planted rose well at least once a
week for the first year, even if its been raining.