The only Lee die for the 6.5-284 that I can find listed is a factory crimp die.
So, remove about 0.04 inches from the base end of the collet part of the die, and it should be able to neck-size the 6.5-284 just fine.
I just checked my own 6.5x55 collet die, and a sample 284 case fits within the 6.5x55 collet without contacting the cartridge body.
If the unmodified 6.5x55 collet die is used on a 6.5-284 case, it should size all of the neck except a 0.04" bit at the neck base. This is likely not a problem. Some reloaders advocate partial neck sizing as a method of centering the cartridge in the neck of the chamber.
If the die is modified with removal of metal from the collet, it might be wise to remove the stamping that identifies it as a 6.5x55 die. Use of the modified die with a 6.5x55 case might result in some disappointment.
I failed to account for the shoulder diameter of the 6.5-284 case vs the shoulder diameter reamed into the 6.5x55 collet.
If the 6.5x55 collet die is used to neck size a 6.5-284 case, the shoulder of the 6.5-284 case is likely to mushroom. The effect will be worsened if the collet is shortened as I described above.
An unmodified 6.5x55 collet can work with the 6.5-284 case
if a spacer is put between the shell holder and the bottom opening of the collet. A simple spacer can be a washer with an opening large enough for the case body. The washer-spacer should be about 0.025-0.030" thick.
The result is that about 0.07 of the case neck will be unsized, rather than the 0.04" described above.
--Bob
(With NO regrets, I might add...) 
