You should bookmark this excellent and easy-to-use resource, for reference when you have a question about tube types:
You can enter the number of just about any tube and get the ratings and application data. Using this resource, 6AN8 and 6AN8A are shown as being functionally identical. Usually when a suffix is added or upgraded it represents an change in the tube, which remains back-compatible with the older type. The improvement may be functional, such as higher maximum voltage rating; or it may reflect changes in the design for better durability, resistance to vibration, or other physical characteristics (such as a glass bulb replacing a metal case in 6L6 vs. 6L6G).
This site also includes crossreference data to other tube types as a guide for substitution, including which types are identical or nearly so, which are similar but with differences in ratings or performance, which are similar but with different pinouts, and which are similar but run on different filament voltages.
For greater in-depth research, any of the several tube manuals contain even more information.