You can retrieve the resolving HTML page for the STU3 version
http://hl7.org/fhir/stu3/v3/AcknowledgementCondition/ --> http://hl7.org/fhir/STU3/v3/AcknowledgementCondition/cs.html
and compare that against the R4 version http://hl7.org/fhir/r4/v3/AcknowledgementCondition/cs.html (replace "STU3" with "r4"), and feed them into https://services.w3.org/htmldiff

This example resulted in this: https://services.w3.org/htmldiff?doc1=http%3A%2F%2Fhl7.org%2Ffhir%2FSTU3%2Fv3%2FAcknowledgementCondition%2Fcs.html&doc2=http%3A%2F%2Fhl7.org%2Ffhir%2Fr4%2Fv3%2FAcknowledgementCondition%2Fcs.html
(pink = removed, green = updated, yellow = new).

Not sure if Lloyd has a better suggestion -- http://hl7.org/fhir/diff.html only lists STU3/R4 differences for DataTypes and Resources, not valuesets or codesystems.

Is there a way to find the delta between the two http://terminology.hl7.org/CodeSystem/v3/AcknowledgementCondition VS http://hl7.org/fhir/stu3/v3/AcknowledgementCondition/ i.e, if there is a change in the valuesets.

For STU3, have a look at http://hl7.org/fhir/stu3/terminologies-systems.html

Under the primary tab "Code Systems", select the secondary tab that you're interested in (e.g. Internal (FHIR) or v3 or v2). This lists the codesystem URIs in STU3 -- depending on whether you're looking at a FHIR, V3, or V2 codesystem, Each has a different prefix URI: http://hl7.org/fhir/..., http://hl7.org/fhir/v3/..., or http://hl7.org/fhir/v2/.. respectively.

Similarly under the primary tab "Value Sets", this lists all the valueset URIs in STU3 -- and they are all prepended with http://hl7.org/fhir/ValueSet/.., http://hl7.org/fhir/ValueSet/v3-..., or http://hl7.org/fhir/v2/....

Click through to the codesystems or valuesets to see the Defining URL on the respective page, at the very top.

ok, so where can we find the URI per STU?

If you're implementing using FHIR R4 or above, yes. If you're using STU3 or earlier, you should stick with the URLs published in the earlier releases.

Now that we know there was a major URI change, was/is it planned to provide the HL7 terminology users with a mapping for those changes?