Tables For Cumulative Tally And Carrying Balances Forward?

I have a client that I provide financial modelling services to (using Excel). They have a requirement to start capturing subscriber movements in their SQL DB. how the table should be set-up and how to extract the necessary movements report. This is largely so that I may include these components in some of the financial models that I am working on.

Subscribers are reported as follows:

Opening subs (the prior periods closing balance; or the sum of new sales at point of 1st entry)

+ New Sales (new subs)

+ Upgrades (movement from a lower product package to the associated package)

- Downgrades (movement to a lower product package from the associated package)

- Churn (subscriber losses)

Closing Balance

All transactions are captured against a specific product package, on a specific date (ymd), and for an associated platform (e.g. digital TV, broadband TV, cable TV).

I believe we only need to capture new sales, upgrades, downgrades and churn. And then used a SP to compile the movements behaviour as described above.

So perhaps the table would appear as follows:

Platform
Package
Date
Movement
Value

DTV
PROD 1
2014-11-02
New Sales
8

DTV
PROD 1
2014-11-02
Upgrades
1

[code]....

So I am assuming that given a table such as the above, we could write a SP to produce an output such as (note, below looks at monthly total so will not agree back to sample above which contains only 2 days):

Platform
Package
Movement
September
October
November

DTV
PROD 1
OPEN
600
676
776

DTV
PROD 1
New Sales
92
106
88

[code]....

how one is best to accumulate the balances given that the open date for any given reporting period is in fact an accumulation of all balances since day 1.

How does one typically capture this type of thing in SQL?