You can substitute all the connectors round or square.

The reason there are different types of connector used is normally to prevent cross-connection of different circuits during assembly. If changing over all to the same type you will need to be careful when connecting everything up. A good tip is to get a few pots of Airfix paint and spot the male and female connectors of different circuits with a unique colour.

A couple of other things:

These type of connectors are fiddly to fit especially if you haven't got much free-length in the cable or the groups of cables are bundled tightly together. You will need to strip back some of the harness sleeving to get the access you need.

The connectors shown in your ebay link are not genuine Tyco Econoseal ones. The ones supplied with blue end seals are generally Chinese copies. Genuine Tyco AMP Econoseal connectors have green seals for up 1mm sq. cable and yellow for 1-1.5mm sq. cable and are rated at 10A per pin.

For anything greater than 10A you need '250' series Econoseal connector which will take up to 2mm sq. cable and 19A per pin. These have orange end seals.

If you're going to all the bother of doing this I would recommend getting genuine connectors. It is often cheaper to buy the genuine product rather than a Chinese copy on ebay too.

https://www.mobilecentre.co.uk/index.php?main_page=index&cPath=30_275

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