As to cutting down to a foot high - what Bryant said.
As to moving them -- if this is the start of summer for you it would be a good idea to wait until late fall so that the plants are shutting down and the temperatures are not an issue anymore.
Raspberries have a shallow
root system and are usually not very happy about summer heat unless shaded. When transplanted, the
roots will be extra sensitive because of having been damaged. Also, consider that thinning them out also means allowing more sun to reach the ground and potentially dry out the soil. If there is still heavy mulch present then that's OK.
But, you know, in general, this is how raspberries work:
- first year: uh, did we plant enough?
- second year: okay, maybe
- third year and onwards: hey, does anybody need young raspberry plants? pleeeease?
... So this
should not really be a big problem.