If zoning is configured properly (i.e. paths to SPA and SPB) ESXi you should not really notice an interruption but simply switch the paths to the LUN(s). If you are running very high workloads, you should consider to upgrade during non-peak hours (the pre-upgrade check will determine CPU load on the SPs and warn you if it's too high for an upgrade). I just upgraded a few VNX5400 Unified systems and it took about 3 hours for the file and block OE upgrade.
André