Meaning of "Schemas are changed infrequently, if at all"

"if at all" is an idiomatic phrase whose meaning tends to emphasize a restriction stated in the preceding clause. In this case, the statement is equivalent to:

Schemas are changed infrequently, if schemas are changed at all.

In more explicit terms:

Schemas are changed infrequently, and it may often be the case that a schema is never changed.

This idiom also can take the forms "if ever", "if anything", "if anybody/anyone", etc.

SUPPLEMENTAL:
Mark Beadles' answer is entirely correct. I add this only to clarify how the construction works and how you may extend it in the matter he suggests.

The basic structure is A₁, if not A₂, where A₂ is an adjective or adverb in the same scale as A₁, but more extreme. For example:

• reddish, if not red
• most, if not all
• some, if not most
• sometimes, if not often

If A₂ is a negative, however, the double negative not A₂ looks odd or ungrammatical, and is rewritten as a positive

• few, if *not none ---> few, if any
• rarely, if *not never ---> rarely, if ever

Your example is one of these rewritten double negatives:

• infrequently, if *not not at all ---> infrequently, if at all

The sense in all these is “I assert A₁, but I will not go quite so far as to assert A₂, although I hold it open as a possibility”.

It may make more sense if you reverse the order,

If at all, schemas are changed infrequently.

Rephrasing even further:

If they are ever changed at all, it is done infrequently.