I think you may be confusing the feed and the media URLs. let's break it down. I'll put all the URLs in as it's a useful reference for me.
1. The feed currently in use is http://rss.acast.com/cheapshow . This works OK though you say it's not up to date.
2. The feed you want to move the Store to using is
http://feeds.soundcloud.com/users/soundcloud:users:157831157/sounds.rss
If you want to change the Store to using this you must follow the first method set out in the page I linked to and which you quoted from above. This will move the Store (provided the feed you are moving to is OK, more on that in a moment) but it will not move subscribers. To do that you need to add the ''itunesnew-feed-url" tag to the acast feed - if you can. You may not be able to, in which case you can't automatically move your subscribers.
3. So far so good, however the Soundcast feed is currently not usable. Just looking at your top episode, the URL given in the feed for the media file is:

(I've had to post an image of it as the forum software keeps truncating it; of course there are in practice no line breaks in it). You can see for yourself that it's incorrect as you have http:// in there twice - in fact it's two URLs glued together and obviously not going to work. The actual URL of the media file is
http://feeds.soundcloud.com/stream/251677311-cheapshowpod-cheapshow-episode-17-b atmans-anger-management.mp3
so you need to change the feed to using that URL, and the equivalent for every other episode. The URL works but forces a download, so you need to deal with that. If you have already applied the method linked to above I don't know where you go from there, you would need to take that up with Soundcloud.
4. You should not try to change the Store to the Soundcloud feed until you have corrected the problems and tested it by subscribing in iTunes, and by entering the URL of one of the media files in a browser and making sure it plays there, not downloads. The reason the Store keeps changing back to the acast feed may well be that when it evaluates the Soundcloud feed it finds it's not working and hence removes it.