SVA1000X or SSA3000X is just the generic reference for the model series.
For example, the same FW is used for SSA3021X and SSA3032X. Hence, to make the naming universal, it is referred to as SSA3000X.
With the SVA, it used to be different: there were different FW files for the SVA1015X and bigger models (SVA1032X, SVA1075X, etc.). Note that I am writing out of my head, so some numbers may be wrong. Also, as you may have noticed, the SVA1015X cannot be upgraded in terms of frequency range.
At some point, though, Siglent unified the FW for the SVA models and this is why it is now called SVA1000X: it works on both the SVA1015X and SVA1032X.
However, it means that the SVA1000X FW has probably more changes to it and may include some additional checks. I would not recommend this as the starting FW for any cross-grade, unless proven to be OK (I don't know, I don't own a SSA-XP or SVA). If the device is already cross-graded, it should be OK to upgrade.