All of the interest a savings bond has accumulated over the years suddenly becomes taxable in the year it reaches final maturity, even though you are not notified in any way of this event by the Treasury. Thus, a forgotten savings bond in a drawer or file someplace could represent a large, unreported tax liability for you as well as an interest-free loan you are making to the government. If you have failed to report income from a mature bond at any time in the past, you must file an amended return for the year in question. Consult a tax adviser. On the other hand, if you anticipate your bonds' final maturities, you can use rollovers into HH bonds to manage your tax liability for any given year.
Bonds reaching final maturity often have large amounts of accumulated interest that is taxable.
You can postpone paying a little longer on the bonds you elect to convert. If you have several bonds reaching final maturity in the same year, you may want to pay off the tax obligation on some and convert others to HH bonds, on which you will pay taxes only for the new interest they earn until the bonds mature or you redeem them.