Depends, what size is radiator compared to towel rail?
I don't know your DIY skill so I don't know how hard it will be for you.
Things to bear in mind are, when you remove rad you may have to make good behind it as the rad would have covered brackets etc and the towel rail is see through.
Also, how far off the wall will the towel rail sit? (on its brackets) it probably is further then the old rad, so pipe work is more then likely has to be adjusted to suit new installation. This means draining down heating system.
When fitting towel rail it may be handy to have another person as they can be tricky to line up if it's your first time.