Hi, Berta. The answer depends on your shellac. Most shellac contains about 5% wax and the wax makes it difficult for other finishes like varnish to bond to it. You can buy dewaxed white/clear shellac but dewaxed orange shellac is hard to impossible to find. You can try dewaxing orange shellac by letting it set in a closed clear jar and the wax will settle to the bottom. You will see a white layer as it begins to settle but this could take some time. You can mix yourown shellac, but you will still have the wax. I suggest that you get the book "Understanding Wood Finishing" by Bob Flexner for more on this subject.
My other comment is that, while orange shellac does not have the resistance to water, alcohol and abrasion as poly, it is still a darned good finish. It lasted for you for 30 years, for instance. And it is a heck of a lot easier to refinish than poly. One other comment on using it though. Make sure that it is fresh. Shellac ages in the can and can become unusable. If it is anywhere close to the last date for sale on the can, find newer shellac somewhere else.