How to run a subquery based on results of a query SQL

I have 2 queries i'd like to run. The idea here is to run a query on the transaction table by the transaction "type". Based on these results, I want to run another query to see the customers last transaction based on a specific type to see if the service ID was the same. If it's not the same, I want to flag it as "upgraded"

Here is the initial query that Pulls the results from a transactions table based on a transaction type:

Select customerid, serviceid
from Transactions
where (dtcreated > @startdate and dtcreated < @enddate) and (transactiontype = 'Cust Save')

The output for this is:

Customerid          ServiceID
    1                   11
    2                   21
    3                   21
    4                   11
    5                   12
    6                   11

What i'd like to do next is run this query, matching the customerID to see what the customers last charge was:

Select serviceID, MAx(dtcreated) as MostRecent
From Transactions
Where (transactiontype = 'Cust Purchase')
Group By serviceID

My Final output combining the two queries would be:

Customerid          ServiceID          Last Purchase      Upgraded?
    1                   11                  11               No
    2                   21                  11               Yes
    3                   21                  12               Yes
    4                   11                  10               Yes
    5                   12                  12               No
    6                   11                  11               No

I thought this might work but it doesn't quite give me what I want. It returns too many results, so the query is obviously not correct.:

Select serviceID, MAx(dtcreated) as MostRecent
From Transactions
WHERE     Where (transactiontype = 'Cust Purchase') AND EXISTS
                          (Select customerid, serviceid
                           from Transactions
                           where (dtcreated > @startdate and dtcreated < @enddate) and (transactiontype = 'Cust Save'))
GROUP BY serviceid