IF UPDATE([UserId]) AND EXISTS (SELECT [UserId] FROM inserted EXCEPT SELECT [UserId] FROM deleted)
BEGIN
    UPDATE [MyTable] SET [UserIdModified] = SYSUTCDATETIME() FROM [inserted] WHERE [MyTable].[Id] = [inserted].[Id]
END

IF EXISTS (...)
BEGIN
...
END

DROP TABLE IF EXISTS [MyTable]
GO
CREATE TABLE [MyTable] ([Id] INT IDENTITY NOT NULL PRIMARY KEY, [UserId] INT, [UserIdModified] DATETIME2)
GO
CREATE TRIGGER [trgMyTableUserIdModified]
ON [MyTable]
AFTER INSERT, UPDATE
AS
BEGIN
    IF UPDATE([UserId]) AND EXISTS (SELECT [UserId] FROM inserted EXCEPT SELECT [UserId] FROM deleted)
    BEGIN
        UPDATE [MyTable] SET [UserIdModified] = SYSUTCDATETIME() FROM [inserted] WHERE [MyTable].[Id] = [inserted].[Id]
    END
END
GO
INSERT INTO [MyTable] ([UserId]) VALUES (1)
SELECT * FROM [MyTable]

WAITFOR DELAY '00:00:00.010'
UPDATE [MyTable] SET [UserId] = 2
SELECT *, 'Updated.' [Time should be] FROM [MyTable]

WAITFOR DELAY '00:00:00.010'
UPDATE [MyTable] SET [UserId] = 2
SELECT *, 'Same as above.' [Time should be] FROM [MyTable]

WAITFOR DELAY '00:00:00.010'
UPDATE[MyTable] SET [UserId] = NULL
SELECT *, 'Updated.' [Time should be] FROM [MyTable]

WAITFOR DELAY '00:00:00.010'
UPDATE [MyTable] SET [UserId] = NULL
SELECT *, 'Same as above.' [Time should be] FROM [MyTable]

WAITFOR DELAY '00:00:00.010'
INSERT INTO [MyTable] ([UserId]) VALUES (1)
SELECT *, 'Same as above for 1st element.' [Time should be] FROM [MyTable]

WAITFOR DELAY '00:00:00.010'
UPDATE [MyTable] SET [UserId] = 2 WHERE [Id] = 2
SELECT *, 'Updated only for the 2nd element.' [Test] FROM [MyTable]