Can we update the primary key of a vertex in gremlin graph?

I have a requirement where I need to update the primary key of a vertex in gremlin graph. I have a 'policy' vertex and 'policy_number' is the primarykey of the vertex. Existing policy number value is '/a123' but I want to delete the special character in the policy_number and keep it as 'a123' with out affecting its connected edges and vertices. In tinkerpop graph documentation I came across mergeV() function but I don't have too many examples to try it. Can some one help with any useful functions in gremlin that is suitable for my requirement.

g.mergeV(["policy_number":"\XXXXXX"]).option(Merge.onMatch,sideEffect(property(single,"policy_number","84XXXXXX")).constant([:]))

but I got the below error

> error: illegal start of simple expression
>
> g.mergeV(["policy_number":"\84XXXXXX"]).option(Merge.onMatch,sideEffect(property(single,"policy_number","84XXXXX")).constant([:]))

First thing, Im not sure if above syntax updates the policy_number with out creating a list of policy numbers as its value and create only one value and also not affecting its connections (edges,vertices).

Instead of mergeV() if there is any other alternative?

If your primary key is actually just a property, and not the node ID, a simple property step should suffice. The mergeV step is quite new and it's possible that the database you are using (I assume DataStax from the tags) does not support it yet. You should be able to use something like this:

g.V().has("policy_number","\\84XXXXXX").
  property(single,"policy_number","84XXXXXX")