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Just for fun (and because I'm in the middle of testing Antimony for stuff
like this), I created the following model in an Antimony file, which I
think expresses what it is you want:


a -> b; k1*a;
k1 = 5;
at(time>=12): k1=6;


and then converted it to SBML:

<?xml version="1.0" encoding="UTF-8"?>
<sbml xmlns="http://www.sbml.org/sbml/level2/version4" level="2" version="4">
<model id="__main">
<listOfCompartments>
<compartment id="default_compartment" size="1"/>
</listOfCompartments>
<listOfSpecies>
<species id="a" compartment="default_compartment"/>
<species id="b" compartment="default_compartment"/>
</listOfSpecies>
<listOfParameters>
<parameter id="k1" value="5" constant="false"/>
</listOfParameters>
<listOfReactions>
<reaction id="_J0">
<listOfReactants>
<speciesReference species="a"/>
</listOfReactants>
<listOfProducts>
<speciesReference species="b"/>
</listOfProducts>
<kineticLaw>
<math xmlns="http://www.w3.org/1998/Math/MathML">
<apply>
<times/>
<ci> k1 </ci>
<ci> a </ci>
</apply>
</math>
</kineticLaw>
</reaction>
</listOfReactions>
<listOfEvents>
<event id="_E0">
<trigger>
<math xmlns="http://www.w3.org/1998/Math/MathML">
<apply>
<geq/>
<csymbol encoding="text" definitionURL="http://www.sbml.org/sbml/symbols/time"> time </csymbol>
<cn type="integer"> 12 </cn>
</apply>
</math>
</trigger>
<listOfEventAssignments>
<eventAssignment variable="k1">
<math xmlns="http://www.w3.org/1998/Math/MathML">
<cn type="integer"> 6 </cn>
</math>
</eventAssignment>
</listOfEventAssignments>
</event>
</listOfEvents>
</model>
</sbml>

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