You cannot change an PAL format PS2 to NTSC. But you can mod your console by installing a modchip like "MAGIC 3.1 ModChip", "DMS3 V2.0 ModChip", etc. in your console.
The modchip will enable you to run and play Your regular PAL format PS2 and PS1 game tiles as well as NTSC format as well as your backup game titles in DVD-r, CD-R, etc. Beside the Modchip will also enable you to watch any DVD movie from any region without the worry of DVD region codes i.e. you can esaily play ur imported DVD's in your PS2.
But please bear in mind that if you install a modchip and play ur backup games, its not illegal but installing a modchip and playing pirated games is ILLEGAL. Besies Sony does not support modding so your warranty will be void. Also installing a modchip requires to sholder 5 to 22 wires in the mainboard of PS2 which should be done only by a very good qualified person or ur PS2 will end up in a dirtbag. Also note that a PS2 modchip is version dependant. So a modchip for V5 PS2 will not work on V7 PS2 and vice versa. So if you install a modchip make sure its compatible with your version.
For more information on modchips and installations,
goto *www.modchip.com/ps2.
For buying a modchip,
goto *www.baazee.com and search for modchips.