If the old TV is not too old it may be able to accept NTSC (525/60) which has a higher vertical refresh rate and thereby makes the flicker less visible. The downside is that 525/60 resolution is even less than PAL (625/50).
If the TV is even older it will not be able to decode the colour part of NTSC and will produce a black and white image with 'dots' in the coloured areas. It may also have a reduced height picture - you won't have to worry about vertical overscan
If the TV is really old then the picture may roll vertically and will need a tweak of the vertical hold control (if an externally accessible one is provided.)
The insides of televisions are dangerous places. Never take the back off unless you really know what you are doing.