Corinne, 

You ask great but complex question. In general, once you can see the wound base then you can more appropriately stage a previously unstageable pressure injury. It would be appropriate to evaluate and stage the wound at each visit. The NPIAP in 2016 modified and clarified some of the definitions of the various Pressure Injury stages. Per the NPIAP, Stage 2 Pressure Injury definition says "adipose tissue is not visible and deeper tissue is not visible", and goes on to say "granulation tissue, slough, and eschar are not present." (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5098472/). So accurate staging really is about what tissue you can see or what tissue is involved and until you can see the base then you don't really know. 

As for DTIs, this is  also a complex matter. DTIs have their own codes in the L89 code set in ICD-10. Knowing how to code them can also be complicated. This article from 2016 in the International Wound Journal says :

• If necrotic tissue develops, the wound should be staged as a stage III, IV or unstageable as is applicable retaining the DTI labelling in the record for root cause analysis and continuous quality improvement.
  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7950046/
  
  
  So there are times when might stage DTIs as well.  I know this is not as clear cut as you would like but when in doubt document the tissue you see and stage appropriately. 
  
  
  Mike White