Question on finding replacements for Diodes

It all depends on what the diode is used for . If simple power supply circuits usually any diode that has a PIV equal to or greater than the origional and the same for the current rating, can be used. Sometimes the speed of the diode is critical, in RF circuits it may be some other parameter that is the critical part.

If you can and the price is OK. Otherwise use an equivalent.

Sometimes now the diodes that SHOULD have been used can be. Heath used to use matched diodes in some ham sets. Now we can buy Schottkys.

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Yes, that is often the case. Many times the circuit is designed to work with the least expensive parts that will meet the specifications. Then later the price of the parts will drop or improved versions will come out. As you mentioned Heath, I had a 30 Mhz counter from them that later I subsituted a differant IC and it went to around 50 Mhz.