I upgraded my observatory and pier last year. My original pier was anchored with bolts secured while I poured the concrete (a frame was used to ensure correct location while the concrete set). With the upgrade I had to cut out two of these bolts and add four new ones. The local Boltco shop recommended Chemset which is apparently very popular in the construction industry.
I drilled the holes through a template that matched the base of the pier into the concrete with a rotary hammer drill. I sweated over ensuring that all was square. The Chemset was really easy to use with care needed to make sure that everything was clean and completely free of concrete dust afterwards. The bolts are within +/-0.5mm of my desired location. I'm very happy with the result and the pier is extremely solid.
If you do go down this path, a rotary hammer drill makes life significantly easier, much better than a regular hammer drill. Like a hot knife into butter
.
Regards,
Chris