Nationwide said its research showed savers want more flexibility and found there was confusion over the current rules.
Despite the desire to transfer from stocks and shares Isas to cash Isas, three in five people over the age of 45 are unaware that they cannot do so, Nationwide's survey found, ahead of Wednesday's Budget.
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Some 12 per cent of the UK's over-45 population with a stocks and shares Isa have considered transferring to a cash version, even though it is not allowed.
Nearly a third of them (31%) said they wanted greater flexibility to manage their finances, which they believe a cash Isa can offer.
Of those who hold both types of Isa, 30% of over 45s hold more than 50% of their Isa savings in a stocks and shares Isa. Nearly one in ten have also used up their full stocks and shares Isa allowance for the 2013/14 tax year.
Graham Beale, Nationwide Building Society's chief executive, said the rule change would be particularly helpful for those older savers approaching retirement who have stocks and shares Isas but want more flexibility about how they manage their capital and access their cash.
He said: "Certainty of income and protecting capital is hugely important for older savers, particularly where it is a vital part of their retirement income.
"So, it is no surprise to see our research show that while one in five older savers have a stocks and shares Isa, more than half of them have thought about transferring to a cash Isa."
Meanwhile, Leeds Building Society chief executive Peter Hill is calling on George Osborne to increase the annual subscription permitted for cash Isas to £25,000. 

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