I'm wondering why you are putting on new hammer shanks. Does this mean you are hanging new hammers? Reinstalling the old ones on new shanks?
The biggest issues with old shanks are going to be a flattened, worn knuckle, or a flange with either too much or too little friction. Either can be solved. In your case, the knuckles are surely flat.
You can remove the old knuckles with heat and glue in new ones, and you can repin the flanges if necessary. Pianotek.com (you'd have to have an account) has many different shank types, including Tokiwa adjustables. You should measure the knuckle to center distance carefully. Is it 15, 16, or 17mm? If the new part is only off by a mm, it will change the action ratio and feel. You don't want to put on the wrong part and end up with a heavy action, or one you can't adjust other parts to. If, for example, the old knuckles are at one distance and the new ones at another, you may not be able to adjust the jacks properly. It's all inter-related.
When is it smart to not do either? When the hammers are shot. I wouldn't hang new hammers on old shanks. You may want to send end samples from each section to Renner or another shop and ask them to match as well as possible and pick the best hammer for the piano. Probably something not too heavy for an older piano. They'll do everything--drill, mount, prevoice the hammers. You just screw them on and regulate the piano.
We don't know your level of knowledge, but just beware: when you start in messing with things in a grand action, it can quickly become an (expensive) can of worms.
A full set of shanks will run you $500, new hammers on shanks more than double that. You might, for example, put on new hammers and shanks, but you're not done yet. Are the strings original? Then new hammers alone might not be a great investment by themselves. You probably need a restringing job for it to sound its best. But wait, there's more.....what about the bridges? Are they cracked? Does the soundboard have any crown left?