You may however "amend category"...
Yay, i'm going to make my cat c aeroplane a cat b so i can have the lower minima! then ill make it a cat e so i can get the circuits to myself!
I'll make sure i mention i read it on pprune
If you really want to get technical, it would be "centre, xxx request change of flight
rules to IFR"
or
"centre, xxx departed Darwin 59er, 136 miles Alice Springs, 9000 feet, proceeding IFR, Alice 0324 request traffic"
Upgrading to IFR sounds much better