Risks of Opening iWork '09 files in iWork 2013?
Here is the list published by Apple.

Changes that occur when exporting to iWork ’09 

All Apps

Curved shadows and contact shadows are converted to drop shadows. 
Comments on objects are removed. 
Anchored objects are converted to floating or inline objects. 
The following formulas are removed: INTERSECT.RANGES, POLYNOMIAL, SERIESSUM, UNION.RANGES, WEIBULL. Note: The last calculated values are exported. 
Interactive charts are converted to regular charts. 
Bubble charts are converted to scatter charts. 
Vertical text—such as Chinese, Japanese, and Korean—isn’t fully supported in iWork ’09. 


Pages

Tables of contents are converted to plain text. They won’t update automatically in Pages ’09. 
Right-to-left text—such as Arabic and Hebrew—isn’t fully supported in Pages ’09. 


Numbers

If a document contains "OR" filters within a column and "AND" filters across columns, all filter rules are exported; "AND" filters are converted to "OR". 
Duration-based filters and conditional highlights are removed. 


Keynote 

The following transitions are converted to Dissolve: Clothesline, Object Cube, Object Pop, Object Revolve, and Switch. 
The Object Flip transition is renamed Revolve. 
The following build-in effects are converted to Dissolve: Blur, Drift and Scale, Fade and Scale, Twist and Scale, Fireworks, Skid. 
The following build-out effects are converted to Dissolve: Blur, Crumble, Fade and Scale, Skid, Twist and Scale, Vanish. 
Emphasis builds are removed. Emphasis builds are Blink, Bounce, Flip, Jiggle, Pop, and Pulse. 
Comments on table cells are removed. 



I think I got the previous version so here is the newer one-moving from iWorks 09 to iWorks 2013 as of April 1. 2014


Changes that occur when importing from iWork ’09 

All Apps

Rotation is removed from tables and charts. 
Table fills are removed. 
Grouped wedges in pie charts are ungrouped. 
OS X search (Spotlight) metadata is removed.  


Pages

Change tracking is only supported for body text. Other tracked changes are accepted as final. 
Linked text boxes are unlinked. 
Links to other Pages files are removed. 
Bookmarks are hidden in Pages for Mac, but if you export your document back to Pages ’09, they'll reappear.


Numbers

Table categories are removed. 


Keynote 

The “Require password to exit show” setting is removed. To set a new password, choose Keynote &gt; Preferences &gt; Slideshow, and then select “Require password to exit slideshows.” Enter a password when prompted, and then click Set Password. This password will apply to all presentations played on this computer. 
Slides indented more than six levels are promoted to level six. 
Links to other Keynote files are removed. 
Rotation is removed from object placeholders. 
The following transitions are converted to Dissolve: Blur, Burn, Falling Tiles, Flash, Motion Dissolve, Radial, Shutter. 
The Revolve transition is renamed Object Flip. 
Convergence builds are converted to Dissolve. 
The following types of builds are removed: Builds on master slides, smart builds, builds on chart legends.