The question: Would it work or will it break something?
I have an old Laptop (Acer Aspire ES1-311) that still works fine for most use cases I have for it, but its wifi is pretty outdated, so I was thinking if it could be replaced? If I remember right from the last time I opened the Laptop, the antennas are located so oddly that it is hard to remove them altogether. It would be far easier, if I could just replace the card and reuse the old antennas..
There are no 802.11n antennas.
But there are antennas for 2.4 GHz, for 5 GHz or both.
I would only upgrade the wifi card if you constantly run into 802.11n performance limits and AC or AX would make a large difference. Most laptop use cases will not see a large user experience difference.
Processor, memory, storage and I/O will stay the same even if you would be able to upgrade to the newest AX card.
It's not quite that easy...
Modern WLAN cards are no longer mini-PCIe (yours is half-height mini-PCIe), but M.2 (various different keying and different length). Theoretically adapters aren't specs conformant, but you can nevertheless find some on Jack Ma's market place, but that comes at a cost - sandwich, making adapter + card significantly thicker, so it's unclear if you can fit it physically. Another issue is that with M.2, the antenna connector got smaller, so your old ones won't fit - meaning you would have to replace the antennas (dig them out of the screen bezel, etc.)...
iirc, some very early 802.11ac Intel WLAN cards (don't ask me for the model number) were still (optionally) available in mini-PCIe configuration, so that's the newest you can fit 'easily'. Furthermore the built-in antennas of your device might be 2.4 GHz only, given that it is currently only equipped with a 802.11bgn card (some notebook manufacturers put better, 5 GHz capable, antennas into the chassis, but that's not guaranteed).
they still make mini-PCIe even for wifi6e 
https://www.aliexpress.com/item/1005004069956401.html
What chipset/ kind of hardware is that? (aliexpress isn't displaying the page for me in .de)
it's an AX210, @ 22€.
Depending on where OP lives, the MPE-AX1800H might be a better option, it's MT7921 based, $15 at Newegg.
@ThiloteE make sure your laptop doesn't have a wifi card white list, other brands do.
Thank you all for the fast feedback. The MPE-AX1800H sounds like an option. I also found this MPE-AX3000H Wlan 6 Mini PCIe card. I am from Germany too.
I have not found a wifi card white list, but that obviously does not mean it does not exist.
So, if I try one of those and it turns out the existing antennas are 2.4 GHz only, will they be fried? or will the mini-PCIe card be fried? Or will I simply miss out on the 5 GHz band, but can use the 2.4 GHz band?
It appears to be an intel AX200.
If you ever consider using it as AP, the 1st one's a better choice.
@ThiloteE this is a common upgrade for old laptops. I have done it many times with great results. Antennas don't seem to matter.
