Taking a recent call I did to a more extreme that it actually was: 

Patient that is symptomatic bradycardic with a valid MOH DNR provided by the sending facility. Patient meets aid to capacity for making her own decisions. After explaining what is currently happening to her and ensuring she is able to make an informed decision, she verbalizes to the crew that she desires intervention for her bradycardia (e.g. atropine/pacing/etc) hopefully to bridge her to a more permanent pacemaker, however should her heart stop she still wants nothing further done at that point. 

Can a paramedic honour a partial rescinding of the DNR like that? Is it an "all or nothing" thing even after the patient clearly stating exactly how she would want each scenario to play out? 

Please see the Ministry of Health Do Not Resuscitation Standard Training Bulletin (here) for full details. The bottom line is a patient (if capable), or SDM, can rescind the Do Not Resuscitate order at any time. 

BLS

Bradycardia, Capacity, Do Not Resuscitate

https://www.ontario.ca/laws/statute/96h02