Upgrading Domino R5 server to R6.5(Clustering)

Hi I am upgrading domino r5 to r6.5,i am having three servers in clustering ,i just want to upgrade only one hub server to 6.5 and wanted rest 2 servers will remain on R5 only.

Is this will effect any impact on clustering/or i will face any issues.

Please give me some valuable suggestions.Thanks in Advance

Dali