I want to replace the common part in the equation by different value. Here is my equation

    -(3/(2*θ)) - (1/2)*Sqrt[9/θ^2 - (2*(-18 + θ))/(3*θ^2) + (324 - 252*θ - 23*θ^2)/
(3*θ^2*(-5832 + 6804*θ - 702*θ^2 + 73*θ^3 + 54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 + 17*θ^5 + θ^6])^(1/3)) +
(-5832 + 6804*θ - 702*θ^2 + 73*θ^3 + 54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 + 17*θ^5 + θ^6])^(1/3)/(3*θ^2)] +
(1/2)*Sqrt[18/θ^2 - (4*(-18 + θ))/(3*θ^2) - (324 - 252*θ - 23*θ^2)/
(3*θ^2*(-5832 + 6804*θ - 702*θ^2 + 73*θ^3 + 54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 + 17*θ^5 + θ^6])^(1/3)) -
(-5832 + 6804*θ - 702*θ^2 + 73*θ^3 + 54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 + 17*θ^5 + θ^6])^(1/3)/(3*θ^2) -
(-(216/θ^3) + (24*(-18 + θ))/θ^3 - 96/θ^2)/(4*Sqrt[9/θ^2 - (2*(-18 + θ))/(3*θ^2) + (324 - 252*θ - 23*θ^2)/
(3*θ^2*(-5832 + 6804*θ - 702*θ^2 + 73*θ^3 + 54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 + 17*θ^5 + θ^6])^(1/3)) +
(1/(3*θ^2))*(-5832 + 6804*θ - 702*θ^2 + 73*θ^3 + 54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 + 17*θ^5 + θ^6])^(1/3)])]


which looks too complicate. However, this include the common part,

(-5832 + 6804*θ - 702*θ^2 + 73*θ^3
+ 54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 + 17*θ^5 + θ^6])^(1/3)


Thus, I replace this part by A using the command

% /. (-5832 + 6804*θ - 702*θ^2 + 73*θ^3
+ 54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 + 17*θ^5 + θ^6])^(1/3) -> A


However, the output shows that not all common parts are changed into A. I get the output,

-(3/(2*θ)) - (1/2)*Sqrt[9/θ^2 + A/(3*θ^2) - (2*(-18 + θ))/(3*θ^2) + (324 - 252*θ - 23*θ^2)/
(3*θ^2*(-5832 + 6804*θ - 702*θ^2 + 73*θ^3 + 54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 + 17*θ^5 + θ^6])^(1/3))] +
(1/2)*Sqrt[18/θ^2 - A/(3*θ^2) - (4*(-18 + θ))/(3*θ^2) - (324 - 252*θ - 23*θ^2)/
(3*θ^2*(-5832 + 6804*θ - 702*θ^2 + 73*θ^3 + 54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 + 17*θ^5 + θ^6])^(1/3)) -
(-(216/θ^3) + (24*(-18 + θ))/θ^3 - 96/θ^2)/(4*Sqrt[9/θ^2 + A/(3*θ^2) - (2*(-18 + θ))/(3*θ^2) + (324 - 252*θ - 23*θ^2)/
(3*θ^2*(-5832 + 6804*θ - 702*θ^2 + 73*θ^3 + 54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 + 17*θ^5 + θ^6])^(1/3))])]


which still has the common part,

(-5832 + 6804*θ - 702*θ^2 + 73*θ^3 + 54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 + 17*θ^5 + θ^6])^(1/3)


How can I chance all these common parts at the same time?

Part of the problem is that

1/x^1/3 // FullForm
(* Times[Rational[1, 3], Power[x, -1]] *)


That is, powers in denominators are automatically parsed as negative powers. So as noted in Cesareo's answer, you have to find the negative powers as well. I would do the following to help automate this process.

First inspect your expression for sub-expressions that might be the same:

Cases[expr, (_)^(-1/3) | (_)^(1/3), Infinity] // DeleteDuplicates
(* {1/(-5832 + 6804 θ - 702 θ^2 + 73 θ^3
+ 54 Sqrt[6] Sqrt[-324 θ^3 + 306 θ^4 + 17 θ^5 + θ^6])^(1/3),
(-5832 + 6804 θ - 702 θ^2 + 73 θ^3
+ 54 Sqrt[6] Sqrt[-324 θ^3 + 306 θ^4 + 17 θ^5 + θ^6])^(1/3)} *)


(Here, expr is your long expression.) By inspecting just these parts, we can see that every sub-expression of the form $$(\textrm{something})^{1/3}$$ is the same. So, we can do a much simpler replacement rule that requires much less typing:

expr /. {a_^(1/3) -> A, a_^(-1/3) -> 1/A}


Try this

% /. {(-5832 + 6804*θ - 702*θ^2 + 73*θ^3 +
54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 +
17*θ^5 + θ^6])^(1/3) -> A,
(-5832 + 6804*θ - 702*θ^2 + 73*θ^3 +
54*Sqrt[6]*Sqrt[-324*θ^3 + 306*θ^4 +
17*θ^5 + θ^6])^(-1/3) -> 1/A}