I wanted to upgrade my internal storage for my series x

The internal hard drive is not user serviceable, you can however add expansion cards that will expand the SSD storage of the console

any kind of illicit access to the internal hard drive can result in account and hardware bans

I'm planning on running my 360 and XB1 games from an external drive, but because my internet isn't the fastest I wanted to start downloading the Series X upgrades now, ready to transfer to the Series X's internal storage so they'd be ready to play sooner.
I'll make do with the non-Series X games until they've been installed then.

Not really...

Xbox Series X games / upgrades / updates has to be installed onto the Series X internal or expansion card SSD,

You can have Xbox One games on your external hard drive that will get an Series X upgrade but the games will need to be transferred to the Series X and then the Series X upgrade will need to be downloaded onto the Series X SSD,

As an example...

I have Marvel Avengers on my external hard drive that will get the Series X upgrade and I will transfer the game over to the Series X SSD ready to download and update the game to the Series X version when available.

Ok what are you storing them on

If non nvme like ssd or mechanical drive rhen no it can't be installed on it

Either the xbox series x storage cards or internal nvme only

Patched or upgraded games must be stored on internal or storage card

Ir can be stored on external drives but can't run or upgrade from external

Must be moved internallu

1 other question the XBOX series X internal hard drive 1TB can Microsoft allow you to upgrade to 2TB internal storage or say 3TB or 4TB as many new games uses nearly 100 GB each so if you have 10 games the harddrive easily fills up I feel Microsoft should of made the Xbox series X internal harddrive expandable so you can increase the internal storage space of the device please don't confuse with card slots on the back of the console for Seagate expansion drive