Don't look for the incoming Reagan administration suddenly to rip up existing defense programs and start new ones, William R. Van Cleave cautioned yesterday.

Van Cleave, head of President-elect Ronald Reagan's defense transition team, said in an interview he will meet with Defense Secretary-designate Casper W. Weinberger in California today to arrive "at an order of priorities," not to make hard decisions on future budgets and weapons.

"I'm going to brief Mr. Weinberger on the Soviet threat and our options," said Van Cleave.

One reason there can be no firm decisions on future budgets, Van Cleave said is that the Carter administration has not disclosed to the transition team how much extra money it will request for the Pentagon for fiscal 1981.

Although Congress recently approved $160 billion for defense for fiscal '81, Pentagon officials acknowledge this will not be enough because of unanticipated inflation, higher fuel costs and operational expenses imposed by Persian Gulf activities, as well as military construction financed through separate legislation. The Pentagon will need an extra $10 billion to cover all those items, according to some estimates.

Van Cleave said the incoming Reagan team at defense will review the Carter administration's fiscal '81 supplemental request, but added that "a supplemental is not the place to start big new programs."

Major changes in the Carter administration's blueprint will have to be made outside the supplemental, Van Cleave said, starting with the Pentagon's five-year plan for fiscal 1982 through 1986.

The Reagan review, he said, will focus on whether there is enough fiscal 1981 money to make military forces ready for combat. His inside look at Pentagon programs, Van Cleave added, "showed the severity of readiness problems."

"There may be a little money" to start on some new military hardware, Van Cleave said. One such possibility, he added, is to start developing canisters for carrying Minuteman III missiles around.

Van Cleave long has favored digging up to 10 extra silos for each of the 550 Minuteman III missiles now deployed. His proposal, which he has described as a "quick fix" to the vulnerability of U.S. land missiles to Soviet attack, is to haul each missile from one silo to another to make it harder for Soviet gunners to keep the missile targeted.

The Pentagon has studied the mobile Minuteman concept several times and has rejected it. One argument against the proposal is that it would take longer to obtain the privately owned land for the extra Minuteman silos than to deploy the new MX missile on public lands in Nevada and Utah. The first MX missiles would be ready by 1986 under the Carter administration plan.

Although Van Cleave has been Reagan's top man in the Pentagon -- he has been reviewing bundles of policy papers in an office down the hall from Defense Secretary Harold Brown's office on the third floor E-Ring -- he said yesterday he has not been asked to fill any permanent job in the Reagan administration yet.

He said his preference would be a policy job at either the Pentagon or National Security Council. Then, with a laugh, he quipped: "I'm making so many enemies around here, I'll probably end up at the Bureau of Indian Affairs."