How to change the variables Qi's to Ri's in one or two steps

I was wondering if there is a command in Mathematica to change all the following Qi's to Ri's? For example, change Q1 to R1, Q2 to R2, etc.

 Q1 a1 + Q2 a2 + Q3 a3 + Q4 a4 + Q5 a5 + Q6 a6 + Q7 a7 + Q8 a8 + 
 Q9 a9 + Q10 a10 + Q11 a11 + Q12 a12 + Q13 a13 + Q14 a14 + Q15 a15 + 
 Q16 a16 + Q17 a17 + Q18 a18 + Q19 a19 + Q20 a20 + Q21 a21 + Q22 a22 +
 Q23 a23 + Q24 a24 + Q25 a25 + Q26 a26 + Q27 a27 + Q28 a28 + 
 Q29 a29 + Q30 a30 + Q31 a31 + Q32 a32 + Q33 a33 + Q34 a34 + Q35 a35 +
 Q36 a36 + Q37 a37 + Q38 a38 + Q39 a39 + Q40 a40 + Q41 a41 + 

 Q42 a42 + Q43 a43 + Q44 a44

I could go through and change the Qi's by hand, but I think there should be a more efficient way to do this.

Thank you.

Answer

I believe I would use:

expr = Q1 a1 + Q2 a2 + Q3 a3 + Q4 a4 + Q5 a5 + Q6 a6 + Q7 a7 + Q8 a8 +
    Q9 a9 + Q10 a10 + Q11 a11 + Q12 a12 + Q13 a13 + Q14 a14 + 
   Q15 a15 + Q16 a16 + Q17 a17 + Q18 a18 + Q19 a19 + Q20 a20 + 
   Q21 a21 + Q22 a22 + Q23 a23 + Q24 a24 + Q25 a25 + Q26 a26 + 

   Q27 a27 + Q28 a28 + Q29 a29 + Q30 a30 + Q31 a31 + Q32 a32 + 
   Q33 a33 + Q34 a34 + Q35 a35 + Q36 a36 + Q37 a37 + Q38 a38 + 
   Q39 a39 + Q40 a40 + Q41 a41 + Q42 a42 + Q43 a43 + Q44 a44;

expr /. S_Symbol :> Symbol @ StringReplace[SymbolName@S, "Q" -> "R"]

Or more restrictively:

rule = "Q" ~~ x : DigitCharacter .. :> "R" <> x;

expr /. S_Symbol :> Symbol @ StringReplace[SymbolName@S, rule]

I feel that operating only on Symbols is cleaner and safer than converting the entire expression to and from a String.

It is not clear to me if you need this, but if you want to do the replacement after (while) Qi's are assigned values you can do it like this:

{Q1, Q2, Q3, Q4, Q5, Q6, Q7, Q8, Q9, Q10, Q11, Q12} = Range@12;

expr = Hold[Q1 a1 + Q2 a2 + Q3 a3 + Q4 a4 + Q5 a5 + Q6 a6 +
         Q7 a7 + Q8 a8 + Q9 a9 + Q10 a10 + Q11 a11 + Q12 a12];

expr /. S_Symbol :> 

  RuleCondition @ Symbol @ StringReplace[SymbolName@Unevaluated@S, "Q" -> "R"]

Hold[R1 a1 + R2 a2 + R3 a3 + R4 a4 + R5 a5 + R6 a6 +
    R7 a7 + R8 a8 + R9 a9 + R10 a10 + R11 a11 + R12 a12]

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