I had SGD as my main currency on Saxo and opened a USD sub account. When selling USD denominated positions on the SGD account, will it still auto convert back to SGD?

Hey man, that's a good question and I've been exploring the Saxo interface for quite awhile now. The answer to your question is: yes, it will still automatically convert back to SGD because that is the base account currency which the holding is held in.

To circumvent this, it's best to follow the instructions in the link below to transfer your holdings to the USD sub-account. This enables you to get away from the 0.75% conversion fee accrued in closing a position within the SGD denominated main account.

https://www.help.saxo/hc/en-sg/articles/3600012...

Depends on the broker. the normal way would be i guess that it rests there as USD in the subbaccount. for every factual currency switch then the banks/brokers often charge avoidable fees.