There are very often discussions about the Ax upgrade regarding supported upgrade paths. The easiest upgrade will be always from Ax 3.0 SP6, but it is technically possible to go from AX 3.0 (any SP) to AX 2009, however:

1. If the LOS layer (cost accounting for example) is involved and you’re upgrading from Ax 3.0 SP5 or earlier, you will need to test the upgrade and you might have to program your own upgrade scripts, otherwise there is a risk of losing the data. We would recommend to go from 3.0 SP6 if any LOS layer is involved.
2. If a country specific DIS layer is involved(any you’re upgrading from Ax 3.0 SP5 or earlier), you will need to go to at least from AX3.0 SP4 due to changes between AX 3.0 SP3 and AX 3.0 SP4 DIS layer data model. Upgrading from directly from AX 3.0 SP3 with for example PL DIS layer will lead to a data loss. As in the point 2#, you can write your own upgrade scripts (or migrate those from SP4 DIS) to prevent the data loss and to be able to go directly from AX 3.0 SP3 to Ax 2009

Write Data Upgrade Scripts white paper (http://go.microsoft.com/fwlink/?LinkId=115169&clcid=0x409)

Technical note:

The reason why we recommend upgrading (to Ax 2009) from Ax 3.0 SP6 is that SP3toSP4 (DIS,LOS) upgrade scripts are in SP6 (or SP4 etc), in LOS or DIS layer. So if you go from SP3->SP6, upgrade scripts in ReleaseDBUpdate30SP*... will take care of everything. However, those scripts are nonexistent in 2009 codebase and this is why we have the problem here.

Martin Ch