The Northern Ireland Assembly has a "seat-switch system" (co-option) that allows for MLAs to be replaced without holding by-elections.
As the BBC explains:
Co-option is the only option when there is a gap on the benches of the Northern Ireland Assembly.
Some Assembly members leave Stormont mid-term for a variety of reasons, and their party selects the successor.
Are there any legislatures that have similar co-option systems that allows for replacements of members without holding by-elections?