Currently, in the USA, an individual can purchase up to $10,000 in electronic Series I ("inflation-protected") bonds in a given year, plus up to $5,000 in paper I bonds. The latter can only be purchased by instructing the Treasury to pay part or all of any income tax refund one may be getting in this form.
Individuals who have purchased paper I bonds in this manner may convert them to electronic form at any time. Suppose one does this, immediately upon their receipt - does this reduce the limit on the number of additional electronic I bonds one may purchase in that year to below $10,000? I do not think so (especially as one may have reached the limit beforehand), but I have not seen this scenario explicitly addressed.