Dependent upon your installation (all the components of the PLC), some of the 1771 stuff has become obsolete or silver series - neither is good for supportability.
I did a PLC2 to PLC5 conversion years ago, used the AB conversion software, and experienced what was referenced earlier. The software does convert a good deal of the code, but does not get it all. One example is the way PLC2 handled analog cards, communications, etc. Stuff like that did not convert over well, or at all. Still took a lot of manual conversion.
Another issue I ran into was that some of the products were NOT compatible. One example was the Chassis. The one for the PLC2 was so old that the PLC5 did not support that revision. Learning point here is to review the parts you have and confirm that they can still be used with the new controller (if that is the route you are going).
For conversion sake, one may want to consider going to the current "Logix" platforms. If you have been following the pricing of "Legacy" hardware (PLC5, SLC, etc), it can be noted that pricing has risen and availability has deminished. This is so customers migrate to the Logix platform. Yes, RA will still support the current products, but new development has ceased long ago.
You may want to do a lifecycle analysis on your equipment. If the equipment is nearing the end of the lifecycle, then plan a conversion that is low cost, but supportable. If the lifecycle is longer, than you should consider the lifecycle of the product you are replacing. Don't put legacy controllers on equipment that is going to have a longer lifecycle. You'll just wind up with supportability headaches and cost, and you may have to do this all over again.