I had the same dillema as the OP. I took my Heckler to the Alps and decided I wanted to get something bigger so instead of spending a lot on my heckler to make it into a mini DH bike I bought a Bullit Frame and built it up for DH duties. This way I can still use my heckler as a trail bike and the bullit for DH.
I figured it wasn’t the right thing to do to spend a significant amount of cash on changing my heckler from a really nice trail bike into something which wasn’t a great DH bike and probably a lot less fun on regular UK trials.
If you are swapping from heckler frame to a new longer travel FR/DH one it is likely that you will need new forks and maybe a 150mm rear wheel which all add up to a fair bit of money.
If I was you I’d do what mike and steve suggest and scour DH forums for a complete older DH bike. It may cost a bit more intially but If you don’t like it you can sell it on without having to mess round with your current bike.
Something like the stinky above or a 222/223/ Giant Glory or Faith can be found for ok money if you are lucky.