First I share my thoughts IBP training content, SOP200 and 300 are good content in SAP Learning Hub, only thing I found actual configuration / setup understanding content to be 30-40% of the total. In fact SOP300 has entire content repeated twice (if you get overwhelmed by 1000+ pages of content) plus there is some overlap between SOP200 and 300. IBP S&OP including Supply should get covered with these two courses. IBP 400 is for Inventory while IBP 500 is Control Tower module / component.
Next regarding APO: Any application transitioning / learning is a 2 pronged approach - process and technology. In that respect SAP PP consultant can transition to APO PPDS as process is similar while technology is different but still can be picked up. With APO DP and SNP not only is process (ready functionalities) is different but technology (read data model, setup & solution approach) is also. Not sure about your journey but I have seen PP consultants struggle initially with APO concepts like liveCache, Planning Area, TimeSeries / OrderSeries, Heuristics, Stat Forecasting, BW Data Mart and whole Planning Model for DP and SNP. I shall limit here as its more of APO space discussion.
Similarly I feel APO DP & SNP consultants would find it easier to transition / learn IBP because the process part is similar and the technology is less of a gap to bridge.
Finally on "or explore (Simple) Logistics in S/4 HANA": You can start with my blog Expectation from Simple Logistics but really you need to deep dive in the S/4 HANA community space
Some good blogs to start on S/4 HANA
http://scn.sap.com/community/s4hana/blog/2015/12/18/christmas-time-time-for-family-friends-and-catching-up-on-s4hana-1511
SAP S/4HANA 1511 - SPS00 : What's NEW & What's NOT !!
SAP S/4HANA 15/11 - Are you ready?
http://scn.sap.com/community/s4hana/blog/2015/12/23/beginners-guide-to-transitioning-to-s4hana-1511-for-existing-sap-erp-60x-users
Note SAP has removed the name Simple Logistics don't worry about S4HANA 1511 version - its yymm nomenclature.
Happy reading and exploring!
Somnath