Insurers treat pre-existing conditions differently depending on the size of the group. For small teams, many plans exclude them for a period, often the first 12 months of cover. Larger groups are more likely to have them covered from the start.
A company that moves from one group plan to another without a gap can often keep cover for employees’ existing conditions, because many insurers accept the previous plan’s cover as continuous.
The exact rules are in each insurer’s policy wording, and they are one of the most important things to compare for a small team.