Carry on reporting exactly as you are

When an announcement says two standards are being combined, one question comes first in practice. Does the reporting we do now have to change?

The answer is: nothing

From the partnership FAQ:

Both organisations' existing standards remain valid until the new standard is published, and further information on the transition period to the new standard will be provided.

If you are calculating and reporting to GHG Protocol today, keep doing that. The point of transition and its length will be set out when the new standard arrives.

It is not a merger

This part is worth stating plainly too. The FAQ says technical experts from both bodies will develop the standard through a collaborative process, and adds:

The final decision on adopting the draft standard remains entirely with each organisation.

Built jointly, adopted separately. "ISO has absorbed GHG Protocol" is not what happened.

Revisions already under way are not discarded

The FAQ states the priority as "building on existing work and minimising disruption to revision processes already under way". Named for harmonisation are the Corporate Accounting and Reporting Standard, the Scope 2 Guidance, and the Scope 3 Standard.

So the Scope 3 revision discussions happening now become inputs to the integrated standard.

Cost

GHG Protocol stated it plans to continue providing the standards free of charge for non-commercial use.

※ This article cites the FAQ published by GHG Protocol.

This article expands one section of LCS international standards analysis 1, Two Standards Become One. The full report works from GHG Protocol's published announcement and partnership FAQ as primary sources, and marks anything still at proposal stage as such.