I have an all on four denture on the lower jaw. Can it easily be changed to a snap in denture? What would be involved?

CAN it be converted? Yes. But not easily. It isn't complicated, but "simple" is not accurate. There are many factors to consider, and the best option is a Deutsch Removable Bridge. 

Acrylic dentures can be modified relatively easily and quickly.  But why would you want to do that ?  

Hi, thank you for your All-on-4 denture question. All-on-4 dentures are prosthesis that are retained in the mouth and normally are much more stable than a Snap in denture. Yes, you can convert from one to the other, but traditionally the All-on-4 prosthesis is better for you. It is a more retentive and stable prosthesis. I think that you should have a conversation with your dental specialist regarding why you want to switch over and weigh the pros and cons with both. Good luck!