[Al Skierkiewicz]

Frank,
You are correct and on the right track. If you are planning a change and the additional parts with your previous robot weight are under 120 lbs, then the additional parts are considered an attachment or additional mechanism which is legal under the robot rules. These mechanisms may be changed from match to match.

R05
The ROBOT weight may not exceed 120 lbs. When determining weight, the basic ROBOT structure and all elements of all additional MECHANISMS that might be used in different configurations of the ROBOT shall be weighed together.

Teams are allowed to make improvements to their robot any time the pits are open, provided the additions are manufactured on site or at the machine shop. If however, the improvement requires removing another part of the robot in order to comply with weight, it may not be exchanged in later matches for the remainder of that event without violating R05. While this might be the first time most of you will be aware of this, the rule has been in place for several years. The interpretation for removing/adding mechanisms that exceed 120 lbs has also been used in competition for many years.
I like to classify this rule as one of the WildStang rules as we often had interchangeable "attachments". Our last foray was in 2003 when we planned a stacker in addition to our other devices. As I remember the stacker was never mounted during competition. During that time, teams merely had to weigh in with their heaviest mechanism to make weight. Shortly there after, it became illegal to swap out attachments that in total exceeded the robot legal weight.