The a1.medium pairs 1 vCPU with 2 GiB of RAM, the 2.00 GiB-per-vCPU ratio that marks a general-purpose instance. In the name, "1" is the generation and "medium" is the size within the a1 family. It runs on AWS Graviton Processor at 2.3 GHz, and AWS introduced it in November 2018. a1 is the current generation of the a family, so there is no newer AWS size to migrate to. gp.vs1.medium is the closest Spot shape: 2 vCPU and 4 GiB, so 1 more vCPU and 2 GiB more memory, about 92% below the US East (N. Virginia) on-demand rate.
Match qualityx86 substitution
a1.medium is Graviton (arm64). Rackspace Spot does not have arm64 servers, so gp.vs1.medium is an x86_64 substitute with similar specs.
You will need x86 builds of your container images. Most software rebuilds without changes, but anything with compiled native code needs a real rebuild rather than a retag. Speed per core also differs between the two chips, so it is worth testing before deciding how many nodes to run.