# How to replace subscript symbols

I have an expression with subscripted variables. I would like to replace all those terms with other symbols. For example,

$P_1 + P_2^3 \rightarrow P1 + P2^3$

I used the following code but it doesn't work

X = Subscript[P, 1] + Subscript[P, 3]^2 - I*Subscript[P, 5]
X /. {Subscript[P, n_] -> Pn}

If I replace Subscript[P, n_] with P[n] then it works (e.g. $P_1$ will be replaced with P[1]) but I don't want the bracket. I want P1 instead. Is there any way to solve this?