I did the transition to Maya in October 2013 when Autodesk had the upgrade to ultimate offer. The move to Maya has not been easy but if you are going to do it, learn the Maya way, trying to force the Softimage workflow on Maya will not work to your advantage in the long run.
[*] Start with the basic (navigation, hot box, menues, hypergraph, scene graph, outliner, channel box, attribute editor, hot keys, etc.)
[*] Choose an area you are confortable with and learn how Maya does it. Expand in your area of expertise first. You are not trying to learn box modeling here (you already know that) but the best way to do it in Maya.
[*] Build on commonalities using your knowledge of 3D environment (coordinate system, MR shadings, transformation and deformers, 3D Math, etc).
[*] As you get confortable with the Maya interface, expand in another area you know. Trust your knowledge and skills.
[*] Learn some basic MEL scripting, Maya is heavily build on this and sooner or later you will need some little scripts to automate your workflow. Python is good as well if you already know how to program in Python.
[*] Be curious, inquisitive and open minded. If you hit an issue, seach the help file and the internet, it's amazing the amount of information on Maya. If you get stuck ask a question on CG Society, The Area or even on this forum. It's often too easy to compare negatively Maya, this is an obstacle to learning that you need to get away from quickly.
[*] Be focussed, you can loose hours in exploring Maya. Put on paper a plan to get where you want to go, set yourself some goals and objectives and stick to it. You will have time to explore later.
[*] There are quite a few videos out there on Maya, but many are too basic for your needs, select and whatch only those that meet your goals/objectives. Many are long winded, skip to the good part if necessary. Keep good notes.
[*] Practice, practice, practice. The aim here is to rebuild you muscle memory to Maya as quickly as possible.
Good luck.
BTW, Softimage 2015 is not a bad version at all. Some good workflow enhancement at first glance.