A totally different approach, excluding JSON, pure SQL

1. create temporary table
2. Insert your result
3. consume it within the same process
4. eventually drop the table definition for a different query

I use the same query for my small code example

SELECT TOP 5 ssn, home_state,name from Sample.employee

USER>
USER>ZN "SAMPLES"

SAMPLES>:sql
SQL Command Line Shell
-----------------------------
[SQL]SAMPLES>>  << entering multiline statement
       1>>CREATE GLOBAL TEMPORARY TABLE %ZZ.resultset as
       2>>SELECT TOP 5 ssn, home_state,name from Sample.employee
       3>>go
       
17.    CREATE GLOBAL TEMPORARY TABLE %ZZ.resultset as
       SELECT TOP 5 ssn, home_state,name from Sample.employee
5 Rows Affected
[SQL]SAMPLES>>quit

SAMPLES>zn "USER"
USER>:sql
SQL Command Line Shell
-----------------------------
[SQL]USER>>SELECT * from %ZZ.resultset
18.     SELECT * from %ZZ.resultset

| SSN | Home_State | Name |
| -- | -- | -- |
| 217-28-9642 | NC | Emerson,Xavier O. |
| 226-71-5540 | CO | Malkovich,Bob V. |
| 396-77-7486 | UT | Joyce,Juanita V. |
| 558-18-5848 | IA | Ingleman,Josephine Q. |
| 943-90-5082 | TN | Page,Valery R. |

5 Rows(s) Affected

Now it's all in your hands
Eventually run DROP TABLE %ZZ.resultset
before next use with different columns.
Or just use different temp table for repeated use with other SELECT

Once upon a time, there was no JSON support!
But your challenge could be resolved by these steps:

1. ZN form original namespace (ONS) to execution namespace (ENS)
2. Execute query in ENS and get ResultSet 
3. While in ENS, let %Display print the Resultset into SPOOL
4. ZN to ONS.
5. Loop over the spooled ResultSet to get your data line by line

A simplified example:

USER>ZN "SAMPLES"
SAMPLES>set sql="SELECT top 5 name, ssn from sample.employee"
SAMPLES>set sta=##class(%SQL.Statement).%New()
SAMPLES>write sta.%Prepare(.sql)
1
SAMPLES>set res=sta.%Execute()
SAMPLES>kill ^SPOOL($j)
SAMPLES>open 2:$JOB
SAMPLES>use 2 do res.%Display()
SAMPLES>close 2
SAMPLES>ZN "USER"

USER>Zwrite ^|"SAMPLES"|SPOOL($JOB)
^|"SAMPLES"|SPOOL(13488,1)="Name"_$c(9)_"SSN"_$c(13,10)
^|"SAMPLES"|SPOOL(13488,2)="Emerson,Xavier O."_$c(9)_"217-28-9642"_$c(13,10)
^|"SAMPLES"|SPOOL(13488,3)="Malkovich,Bob V."_$c(9)_"226-71-5540"_$c(13,10)
^|"SAMPLES"|SPOOL(13488,4)="Joyce,Juanita V."_$c(9)_"396-77-7486"_$c(13,10)
^|"SAMPLES"|SPOOL(13488,5)="Ingleman,Josephine Q."_$c(9)_"558-18-5848"_$c(13,10)
^|"SAMPLES"|SPOOL(13488,6)="Page,Valery R."_$c(9)_"943-90-5082"_$c(13,10)
^|"SAMPLES"|SPOOL(13488,7)=$c(13,10)
^|"SAMPLES"|SPOOL(13488,8)="5 Rows(s) Affected"
^|"SAMPLES"|SPOOL(13488,2147483647)="{67640,77930{9{"
 
USER> 

The content here is:

Name    SSN
Emerson,Xavier O.       217-28-9642
Malkovich,Bob V.        226-71-5540
Joyce,Juanita V.        396-77-7486
Ingleman,Josephine Q.   558-18-5848
Page,Valery R.  943-90-5082
5 Rows(s) Affected
Now you can loop over your result as on any other Global
as you need

ClassMethods can be projected to SQL as Procedures.
That's what I  used in the only class (ISOS.cls) of this package
The package was written for IRIS but worked in Caché as well for ages..

In OEX there are 2 examples to create a persistent (or temporary) class from JSON
Launching a related query to get a ResultSet is just a standard exercise then.

• https://openexchange.intersystems.com/package/JSON2Persistent
• https://openexchange.intersystems.com/package/json2class 

so this acts in some way similar to #(variable + formula)# syntax
that we know from &html<..> or &js< ..> blocks for CSP

??

first you may try to run your query in your code for a few records 
to see by what format EFFDT is returned internally

As my 5* review in OEX is still blocked after fixing 
a minor typo in text I'lll publish it here.
-----------------------------------------------------------------------

list display failed at that point also text was scrambled
now it's working normal again

So it's clear.
Old_Class loads old defined properties in a local Object structure 
and writes only this structure back.

It has no idea of New properties and doesn't write it.
Just a guess:
$LB(...) in ^GlobalD of Old_class is shorter than for New_class
 So %Save() of Old_class is a data killer.
 

OK,
but prerequisit is, that both definitions are identical.
Especially in the Storage definition, where properties are mapped to globals

a selphy ??  😉

ATTENTION ! This made me nervous.

In essence, the two classes are sharing the same data / Index / stream globals.

This means there is also
Parameter MANAGEDEXTENT As INTEGER = 0;
Otherwise, it wouldn't compile