With PSP Slim old Firmware would not work.. Have you knew that SLiM version has TV-Out feature which is being Controlled by Software.. 2nd it has new MoBo chips and parts.. you could brick your PSP in no time..
About Up/Down - you don't need 1.50 firmware.. new firmware mods such as M33 (dont remember correctly) has already made everything in Automated way.. but overall it is risky..
The Most secure way of playing with firmwares would be mod Chip.. It is replacement of your original PSPs memory where Firmware being written.. to Flash.. which could be easily accessed through USB as a simple Mass Storage device.. and with specific unpackers (dont matters bricked your psp or not) you could easily Roll Back... or go for a new Firmware..

Chip is totally worth buying ..
For new SLiM version wait a while.. at least month.. for new Mod Firmware to come to your specific Console. the slim version