The best suggestion I can give you:
1. Make the test with separate answer sheet option
2. AMC will print choices with boxes below questions and make a separate bubble sheet.
3. Pl ask students to write their temporary answers below the questions and bubble the
separate answer sheet only when they are 100% sure.
4. This should take care of your question in 99% of the cases.
5. In rare cases students want to change their final answer on the bubble sheet. In such
instances, pl ask them to write their "final" choice/answer on the bubble sheet itself
-- separately in words like -- "My final answer for Q20 is A". Look at these bubble sheets
and you use "manual" option in data capture mode and edit these answers.

If you use the test version, change the default value of the upper_threshold.

extract of the english documentation :
gives the black ratio threshold over which a box is considered as not beeing ticked. You can set this value to (for example) 0.6 to allow students to cancel a ticked box filling it completely: if the black ratio is between threshold and upper_threshold, the box is considered as beeing ticked, but if the black ratio is greater than per_threshold (for example when the box is filled), the box is considered as not beeing ticked. The default value is 1.0, so that this feature is disabled.

Otherwise the student erases the marks with tippex.

The best and simplest way for students to correct their answers is to use a white correction pen. They should cover the box with white and shouldn't try to redraw the box and its contents.