How do I change my AWS Redis cluster instance type without losing my data within it?
Short Answer - No, Just don't reboot your master nodes.
Long Answer - Elasticache implements somewhat of a replacement work-flow whenever you modify your clusters. New Nodes will be launched in the backend with that modification (in your case, a different node type) and then they sync with the data in your current nodes. Once the nodes are in sync, there will be change in the DNS records and modification will end.
https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/CacheNodes.NodeReplacement.html
PS - always a good idea to take back up if its the cluster has critical data.
Cheers.