R-experts, I have a bunch of files (by date) that I can read into
dataframes as below.

df$today:                                                       
identifier      rtgmdy  rtgmdy_dt       rtgmdy_watch    rtgmdy_nowatch
rtgmdy_watch_dt rtgsp   rtgsp_dt
310000031       Aa3     20050701                Aa3     NA      AA-
20050510
310000086       B1      20070920                B1      NA      B+
20070828
310000106       Baa2    20040326                Baa2    NA      BBB
20051003
310000170       Baa3    20070601                Baa3    NA      BBB+
20051024
310000225       Ba2     20070601                Ba2     NA      BB+
20070423
310000386       Baa3    20070320                Baa3    NA      BBB-
20040331
                                                        
df$20050701:                                                    
identifier      rtgmdy  rtgmdy_dt       rtgmdy_watch    rtgmdy_nowatch
rtgmdy_watch_dt rtgsp   rtgsp_dt
310000031       Aa2     20030501                Aa3     NA      AA-
20050510

I want to re-cast the combination of df$today and df$20050701 as
follows:

PART A: Ideal result: Where I can have just identifiers and dates and
see what changed for that date(s), and also what it changed from. I
realize that it takes a combo of some nifty melt and cast work that my
iterations have thus far not resulted.

identifier      Chg Date        Var1    Var2    Var3=(Previous Var2)

310000031       20050701        rtgmdy  Aa3     Aa2
310000031       20050510        rtgsp   AA-                             
310000086       20070920        rtgmdy  B1                              
310000086       20070828        rtgsp   B+                              

PART B: Less than ideal result (where we don't the lookup to get the
previous value). I'll live with this type of result also. I realize the
lookups can be expensive computationally.

identifier      Chg Date        Var1    Var2
310000031       20050701        rtgmdy  Aa3
310000031       20050510        rtgsp   AA-
310000086       20070920        rtgmdy  B1
310000086       20070828        rtgsp   B+

Any pointers? Thanks!
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