If you have play in your formerly preloaded bearings, definitely replace them. You don't have to even replace them with the highest-rated bearings. A good brand of mid-range bearings is not that expensive and well worth the effort. Check out youtube for some good videos regarding spindle...
There are quite a few videos on youtube about retrofitting a power drawbar to a mill, the belville stack being the most popular. I just bought a used Denford Triton Pro CNC machine with an auto tool change BT30 spindle which uses a belville stack for the drawbar on the ATC (Automatic Tool...