As said by aborzenkov it is not possible to convert single to multi and vice-versa . Have a look at this differences between them.
In a single-mode interface group, only one of the interfaces in the interface group is active. The other interfaces are on standby, ready to take over if the active interface fails. All interfaces in a single-mode interface group share a common MAC address.
There can be more than one interface on standby in a single-mode interface group. If an active interface fails, the cluster randomly picks one of the standby interfaces to be the next active link. The active link is monitored and link fail-over is controlled by the cluster; therefore, single-mode interface group does not require any switch configuration. Single-mode interface groups also do not require a switch that supports link aggregation.
Dynamic multi-mode interface groups implement Link Aggregation Control Protocol (LACP) to communicate group membership to the directly attached switch. LACP enables you to detect the loss of link status and the inability of the node to communicate with the direct-attached switch port.
A dynamic multimode interface group requires a switch that supports LACP.
The following rules apply when using dynamic multimode interface groups:
Dynamic multimode interface groups should be configured to use the port-based, IP-based, MAC-based, or round robin load balancing methods.
In a dynamic multimode interface group, all interfaces must be active and share a single MAC address.