You have not mentioned your number of years of experience in Datawarehousing tools and also it is important to know how much involved you are in OLAP-Cognos, OFA and ETL-Datastage. Is it just a small exposure or involved in depth. Also you must understand is why you want to switch over to SAP-BI. Data Warehousing itself has a good future provided you are technically involved in it and have interest to grow along with it. What IT degree(s) you hold?
Yes, you can learn SAP-BI because you are already involved in high end activities of Data Warehousing (I presume!)
Pre-Requisites to learn SAP-BI shall be good at complex query writing, complex code writing, databases, database design, database structures, understanding of business and business needs, and SAP product.