@garceau.dav
For the future, if you plan ahead you can change an existing chamfer later as follows:
1. click the radio button for Animate mode in the Edit poly rollout
2. select the edges you want and apply the chamfer settings you want
3. (optional, but recommended) Rename the Edit Poly modifier (Right click>Rename) to something like EP_Chamfer
As long as you do not click the Commit button, the chamfer settings will be changeable and available via the Settings button as shown below:
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For the present situation you have, if you had made the chamfer in a single Edit poly modifier with no other changes, then you could at least turn off that edit poly modifier and re do the chamfer with new settings that you want (using animate mode as described above, if you think you may want to change later. If you just made the chamfer and then made lots of other changes in the same Edit Poly modifier, there is no way to go back except by undo. So no way to selectively change the chamfer that you made.
However, in cases where you have a lot of time invested in a model and are just not happy with the chamfer on some edges, I have made changes manually by removing the all the Chamfer edges except the original edge selection that you chamfered (i.e., the middle edge of the chamfer) and then making adjustments to to position of the original edge so that aligns properly with the surface to make a 90 degree angle. Then you can select the same edges and re-do the chamfer as you want. For sure this can be tedious, but it may be worth it depending on your model vs starting at an earlier save version of the model. Hope something here helps you out.
Edit: Here is a link to video demonstrating Animate mode in modeling:
https://www.youtube.com/watch?v=pTI74Au9zPY