You gotta have some drum replacement software like Drumagog in order to do this.
First, you load your old demo up into your trusty wave editor, and using eq, delete all frequencies above 70 hz. Possibly everything above 60hz. Depends on how much low end the bass has, vs. how much low end the kick drum has.
You only want the lowest part of the spectrum for kick drums i.e. the 50-70 hz range.
Note that if the bass is down in that spectrum, this trick won't work.
What you just want is a trigger. The only way to tune everything else on the demo out leaving only a kick drum trigger is to remove all that stuff over 60-70 hz.
Then, you choose a good clean kick drum sample with a lotta highs and high mids, so you can get some good snap on the top end and some punch in the mids, and load it into your drum replacement plugin.
You will probably have to adjust your sensitivity and input settings, but hopefully you can get it so the drum replacement plugin will detect the original kick drum hits, and play the sample simultaneously.
Once you do this, and apply the plugin to the track, you will have a track with the new sound of the sampled kick drum.
It will be the same length as the original song of course, so what you do is now save it as something like kickdrumnew.wav.
Now you open up your multi-track program and load the original song on track one, then load the kick drum track you just created which has all kindsa sparkling top end on track two, and then do a mixdown with however much of the new kick drum track you want in the mix.
Figured this out as I am working on an old song where the kick has no definition at all, and I am remastering the song. Just want to spiff it up a bit, that's all.
Figured I would post this piece of info for all you guys out there who might want to make those old demos with Mississippi Mud kick drums sound a bit better.
Cheers.