IT IS NICE TO KNOW that President Carter supports, in principle, proposals that would lift the ceiling on interest rates available to small savers and also change the way banks and thrift institutions operate. It would be even nicer to know the precise details of those proposals. Details are often as important as principles in tinkering with the nation's financial structure. Unfortunately, the specifics of what the president has in mind won't be available for several weeks. The administration still needs to get its own ideas sorted out and to take soundings on Capitol Hill about what changes are politically possible.

The details involve such things as how the higher interest rates will be phased in and what will happen to the slightly higher rate that has long given savings and loan institutions a slight advantage over banks. Without those, it is hard to judge the impact of the administration's proposals on the financial institutions themselves and on the economy-the housing market, in particular.

The president is quite right in saying the current system "has become increasingly unfair to the small saver." When the rate of inflation is running as high as it has been for many months now, individuals actually lose buying power by putting money in accounts that yield 5 percent or 5 1/4 percent interest. Yet that is the maximum rate existing federal rules permit banks and thrift institutions to pay on funds that are readily accessible to the saver.

There is wide support, among bankers as well as consumers, for either raising that rate much higher or eliminating the federal maximum entirely, as the president has proposed. But there will be disputes over his proposals to permit S&Ls to issue variable-rate mortgages and make consumer loans and to permit all federally insured banks and S&Ls to offer interest-bearing checking accounts.

If these proposals set off a free-for-all on Capitol Hill as the banks and S&Ls maneuver for postion, so much the better. It's time for a general overhaul of the banking and savings business, and this is as good a way as any to begin.