Well, I'm talking specifically about contracts in response to this post. What the USPTO is amending or modifying is different.

But yes, you can modify without an amendment. For example, you can make minor changes like correcting the spelling of a street name or something like that. This is often just handwritten on the agreement. Both parties initial (or otherwise indicate their approval of the change) and then sign as normal.

If allowed in the terms of an agreement, a party can simply change language in the agreement as desired (see your credit card company contract).