I also think if a trader has opened a position already and he chose isolation margin, he will not also be able to convert to cross margin accept when he closes the trade to open new position using cross margin. I hope I am correct about this.
This is going to be exchange specific, for example, I have a feeling bitmex used to allow isolated to cross margin moves (but not backwards).
Try the Fybit platform, I trade there myself, there are no problems.
I'd be careful using a lesser known exchange, at the very least they could have exploitable bugs.