There are a few specs that you need to know. With those you can start searching the web for options:
First is pedestal diameter at the bushing. It is probably 2 7/8 inches but check.
Second is shape and size of base. Some bases are round, some have a flat side . Bolt hole spacing is a factor; you may or may not find the same pattern spacing and therefore need to make new mounting holes.
Finally, your pedestals are probably flat on the bottom. Some adjustables do so by sliding the main post into the space under the pedestal. That probably is not an option
Of course you need to know what your height requirements are. The spec for a pedestal is from the top lip of the tube to the bottom of the base. Adjustibles will spec a range of course.
There are options to extend a pedestal 6 inches. These slip on the post abd get tightened bt a set bolt. I use one but I had to cut it down, 6 inches was too tall.. Downside is that they are not adjustible.
When you have nothing to do, see if you can unbolt or at least loosen the screws on the old base. It may be frozen in plase and can be a pain in the butt job to get the bolts out.
Here is an example of an adjustible pedestal. I am not saying that it is what you need or if it matches your current mount