Data gets changed when copying data in chunks between two identical tables - sql

Question

In short, I am trying to copy data from one table to another nearly identical table (minus constraints, indices, and a precision change to a decimal column) in batches using Insert [NewTable] Select Top X * from [Table] but some data is getting changed during the copy. Read on for more details.
Why we are copying in the first place
We are altering the precision of a couple of columns in our largest table and do not have the time in our deployment window to do a simple alter statement. As an alternative, we decided to create a table with the new schema and copy the data in batches in the days leading up to the deploy to allow us to simple drop the old table and rename this table during the deployment window.
Creation scripts for new and old tables
These are not the exact tables we have in our DB, but they've been trimmed down for this question. The actual table has ~100 columns.
CREATE TABLE [dbo].[Table]
(
[Id] BIGINT NOT NULL PRIMARY KEY NONCLUSTERED IDENTITY,
[ForeignKey1] INT NOT NULL,
[ForeignKey2] INT NOT NULL,
[ForeignKey3] INT NOT NULL,
[Name] VARCHAR(MAX) NOT NULL,
[SomeValue] DECIMAL(14, 5) NULL,
CONSTRAINT [FK_Table_ForeignKeyTable1] FOREIGN KEY ([ForeignKey1]) REFERENCES [ForeignKeyTable1]([ForeignKey1]),
CONSTRAINT [FK_Table_ForeignKeyTable2] FOREIGN KEY ([ForeignKey2]) REFERENCES [ForeignKeyTable2]([ForeignKey2]),
CONSTRAINT [FK_Table_ForeignKeyTable3] FOREIGN KEY ([ForeignKey3]) REFERENCES [ForeignKeyTable3]([ForeignKey3]),
)
GO
CREATE INDEX [IX_Table_ForeignKey2] ON [dbo].[Table] ([ForeignKey2])
GO
CREATE TABLE [dbo].[NewTable]
(
[Id] BIGINT NOT NULL PRIMARY KEY NONCLUSTERED IDENTITY,
[ForeignKey1] INT NOT NULL,
[ForeignKey2] INT NOT NULL,
[ForeignKey3] INT NOT NULL,
[Name] VARCHAR(MAX) NOT NULL,
[SomeValue] DECIMAL(16, 5) NULL
)
SQL I wrote to copy data
DECLARE #BatchSize INT
DECLARE #Count INT
​
-- Leave these the same --
SET #Count = 1
​
-- Update these to modify run behavior --
SET #BatchSize = 5000
​
WHILE #Count > 0
BEGIN
SET IDENTITY_INSERT [dbo].[NewTable] ON;
INSERT INTO [dbo].[NewTable]
([Id],
[ForeignKey1],
[ForeignKey2],
[ForeignKey3],
[Name],
[SomeValue])
SELECT TOP (#BatchSize)
[Id],
[ForeignKey1],
[ForeignKey2],
[ForeignKey3],
[Name],
[SomeValue]
FROM [dbo].[Table]
WHERE not exists(SELECT 1 FROM [dbo].[NewTable] WHERE [dbo].[NewTable].Id = [dbo].[Table].Id)
ORDER BY Id
​
SET #Count = ##ROWCOUNT
​
SET IDENTITY_INSERT [dbo].[NewTable] OFF;
END
The Problem
Somehow data is getting garbled or modified in a seemingly random pattern during the copy. Most (maybe all) of the modified data we've seen has been for the ForeignKey2 column. And the value we end up with in the new table is seemingly random as well as it didn't exist at all in the old table. There doesn't seem to be any rhyme or reason to which records it affects either.
For example, here is one row for the original table and the corresponding row in the new table:
Old Table
ID: 204663
FK1: 452
FK2: 522413
FK3: 11190
Name: Masked
Some Value: 0.0
New Table
ID: 204663
FK1: 452
FK2: 120848
FK3: 11190
Name: Masked but matches Old Table
Some Value: 0.0
Environment
SQL was run in SSMS. Database is an Azure SQL Database.