当前的产品限制，可以作为一个 enhancement 跟踪，Decimal type fields cannot be resized or precision improved within the partition table · Issue #54441 · pingcap/tidb · GitHub
后续产品会逐渐优化这个。限制分区表修改 partition 的 pr 是：ddl: restricting MODIFY COLUMN on partitioning columns. by mjonss · Pull Request #38670 · pingcap/tidb · GitHub