Whenever one house of Congress significantly alters a bill .?

Answer 1

When the actions of the other chamber significantly alter the bill, a conference committee is formed to reconcile the differences between the House and Senate versions. If the conferees are unable to reach agreement, the legislation dies. If agreement is reached, a conference report is prepared describing the committee members' recommendations for changes. Both the House and Senate must approve the conference report.

Answer 2

If a bill is changed it goes back to committee to reconcile the differences in the bill. At this point it can be rewritten, changed to reflect the differences, or tabled. If the bill is revamped it goes back to the congress for a vote.