I have an old EOS 630 which has two Canon EF lenses, a 35-105mm 3.5-4.5 and a 100-300mm 4.5-5.6 Ultrasonic. These lenses still seem to have a value of ~$100 on eBay. It seems the EF can be used on DSLRs. So I'm looking to keep using them if possible.

But I have a feeling there is a conversion factor for focal length as most of the new camera kits standard lens is 18-55mm. 50mm used to be the standard focal length. What's changed?

Many modern dslrs will have a crop factor of sorts. In the case of canon it's commonly 1.6x, meaning a 100mm lens will have the effect of a 160mm lens.

Having said that there are still many canon dslr cameras that are 'full frame' and don't have that crop factor, just depends on the particular model.

I've got a 1.6x crop (1000d) that supports both ef and efs (like many will) and considering I don't do a lot of really wide angle landscape stuff there is not much disadvantage, infact I get a bit of an advantage with my 300mm covering that bit more ground.

You will probably get frustrated if your widest lens is 35mm on a crop (~56mm).

what is changed is most digital SLR cameras don't use a 35mm sensor most use the aps sized sensor which is cropping the printable area equivilent to just cutting away about a third of your negative
you can buy full frame 35mm cameras that don't have this problem but they are generally quite expensive models