SQL change over time query - sql-server-2012

Question

I have created 2 tables. one table has 4 fields. a unique name, a date and 3 figures. The second table contains the same fields but records the output of a merge function. therefore has a date at which time the update or insert function happened. what I want to do is retrieve a sum of either the difference between 2 days or alternatively the totals of the 2 days to work out how much the value has changed over the day. The merge function only updates if a value has changed or it needs to insert a new value.
so far I have this
select sum(Change_Table_1.Disk_Space) as total,
Change_Table_1.Date_Updated
from VM_Info
left join Change_Table_1
on VM_Info.VM_Unique = Change_Table_1.VM_Unique
where VM_Info.Agency = 'test'
group by Change_Table_1.Date_Updated
but this would just return the sum of that days updated total rather than the difference between the two days. One answer to this question would be to to add all new records to the table but this would contain a number of duplicates. So in my head what I want it to do is loop over the current figures for the day then loop over the next day but also to include all values that haven't updated. sorry if I haven't explained this well. so what I want to achieve is to get some sort of change of the total over time. If its poor design im in a position to accept that also.
Any help is much appreciated.
maybe this would explain it better. show me total for day 1, if the value hasn't changed then show me the same value for day 2 if it has changed show me new value. and so on...
ok to further elaborate.
the Change_Table looks like
vm date created action value_Field1 value_field_2 Disk_Space
abc 14/10/2013 insert 5 5 30
def 14/10/2013 insert 5 5 75
abc 15/10/2013 update 5 5 75
so the out put I want is for the 14th the total for the last column is 105. On the 15th abc has changed from 30 to 75 but def hasn't changed but still neds to be included giving 150
so the output would look like
date disk_Space
14/10/2013 105
15/10/2013 150

Answer 1

Does this help? If not, can you provide a few rows of sample data, and an example of the desired result?
select
(VM_Info.Disk_Space - Change_Table_1.Disk_Space) as DiskSpaceChange,
Change_Table_1.Date_Updated
from
VM_Info
left join Change_Table_1 on VM_Info.VM_Unique = Change_Table_1.VM_Unique and VM_Info.Date = Change_Table_1.Date_Updated
where
VM_Info.Agency = 'test'